As of version 5 and later, Photoshop includes a very helpful "Save for Web" command. This element enables you to save a copy of your image that is optimized for internet use. "Optimized" means that the image file will be as small as possible, and that the image will use only Web-safe colors. The "Save for Web" option can save images in GIF, JPEG, or PNG format.
Using Save for Web When your image is ready to be put onto your web page, select "Save for Web"... from the File menu. The "Save for Web" dialog appears:
In this dialog box you have the opportunity to tell Photoshop how you'd like to optimize your image for the Web. You can select the file format to produce (GIF, JPEG or PNG), what size palette you'd like to use, how to cut down the colors to fit the chosen palette size, how much to dither the optimized image (if at all) and how much to sacrifice image quality to produce a smaller file size.
Image views You will see four tabs above the image view pane on the left side of the screen. These options allow you to view the original image, the optimized version (this is the default), the original and optimized versions side-by-side ("2-Up") and the original, optimized and two variations on the optimization all together ("4-Up"). You can use the Zoom tool to zoom in and out of the image; the Hand tool to move the view around; and the Eyedropper tool to select a color from the image which can be used by various color options on the right of the dialog.
Presets There are many options in the Save for Web dialog, and the choices here can be quite bewildering! Fortunately, there are lists of presets you can choose from to make it easier. Select the Settings: drop-down list, which is located just below the Cancel button, to bring up the list of presets:
There are three basic image formats in the presets - GIF, JPEG and PNG. As a basic rule, use JPEG's for photos and GIF's for everything else. For a full tutorial on choosing the right image format, see Understanding image formats.
Optimizing GIFs If you're making a GIF, you will want to start off with a preset such as GIF 32 Dithered, which works well for most GIFs. You can then fine-tune the optimization to fit your needs. Some of the important optimization options are discussed below.
Color reduction method This is the drop-down box located below the Image Format box. The most frequently used options here include:
Perceptual: Creates the reduced colour palette favouring colours that the human eye is more sensitive to. Selective: Like Perceptual, but better for Web work. The favoured (and default) option. Adaptive: Creates the palette by taking an even sampling of colours from the image's colour spectrum. Generally not quite as good as Selective. Web: Use if you need a complete Web-safe colour palette (Netscape 216-colour palette). These days, this usually isn't that important as most displays support at least 16 million colours. Dithering method This is the drop-down box below the "colour reduction method" box. Dithering involves adding patterned or random dots to the image to make it appear to contain more colours than are actually in the palette, allowing you to use a smaller palette size. The options are:
No Dither: No dithering is applied to the image. Good if the dithering effect looks bad, but can produce severe "banding" of colours. Diffusion: Uses error diffusion dithering, which looks similar to Noise dithering, producing a random-looking pattern of dots. Can produce seams when using ImageReady slices. Pattern: Uses a pattern for dithering, rather than random dots. Can work well for some images, but often produces a rather artificial look to the image. Noise: Similar to Diffusion, but does not produce seams at the edges of slices in ImageReady. Transparency This checkbox is only available if your image does not have its Background layer turned on. It specifies that you want parts of the saved GIF to be transparent. If you deselect this checkbox, the transparent areas will instead be filled with the Matte colour (or white if no matte is selected).
Interlaced This controls GIF Interlacing. If enabled, the GIF will appear gradually as interlaced horizontal lines as it is loaded onto the Web page. This gives viewers something to look at while the full image appears. It does increase the file size, however.
Lossy This slider enables you to remove some detail from the image, in order to reduce the file size further. Only use this option if you don't mind reducing the image quality quite severely! A value of 0 will not remove any detail; a value of 100 will remove the maximum amount of detail.
Colors This is where you choose the size of your GIF palette. A palette of 32 colours is often sufficient for web images, but if your image has a lot of detail and looks too fuzzy/blurry/banded with 32, up it to 64, 128 or 256. If your image has very few colours in to start with, or doesn't look too bad with fewer colours, select 16, 8, 4 or even 2! This will make the GIF file size smaller.
Dither If you have selected a dithering method (see above), this option lets you control how much the image is dithered - 0 means no dithering, 100 means lots of dithering.
Matte The matte is the background colour you want to use for your image. If you've enabled Transparency (see above), then the foreground image will be faded at the edges to blend in with the matte colour. If you choose the Matte setting of None, the GIF will have a "hard transparency" with no fading; this is great if you want to be able to use the image on any colour background.
If you've disabled Transparency, the transparent areas of your image will be filled in with the matte colour.
Web Snap If you want to use web-safe colors you will want to increase the Web Snap slider. The higher the value of Web Snap means Photoshop will bias your color palette more and more towards web-safe colors.
Optimising JPEGs If you're making a JPEG, start off with a preset such as JPEG Medium, which should work well for most JPEGs. You can then fine-tune the optimisation to suit your needs. Some of the important optimisation options are discussed below.
Quality There are two ways to modify the quality of the compression: the Low/Medium/High/Maximum list, and the Quality slider (for fine control). The lower the quality setting, the more blurry and bitty the JPEG will appear, but the smaller the resulting file size.
Progressive A progressive JPEG is analogous to an interlaced GIF (see above). The image will load step by step on the Web page - a low resolution image first, then eventually the full, high-resolution image. Again this is great for stopping your viewers on slow modems from getting bored, but it does mean a slightly larger file size, and older browsers don't support progressive JPEGs.
Matte If your Photoshop image has transparent areas, you can fill them with a specified matte color with this drop-down box.
Optimizing PNGs Options for optimizing a PNG-24 are nearly the same as those for optimizing a JPEG. Likewise, the options for optimizing a PNG-8 are very similar to those for optimizing a GIF. See the GIF and JPEG sections above for details.
If you really can't be bothered... ...you can always allow Photoshop to optimize your image for you! Select the little arrow just to the right of the Settings... box and select Optimize to File Size...:
In the dialog that pops up, choose Auto Select GIF/JPEG then enter your desired file size. Click on OK and Photoshop will do the rest for you! If you're not pleased with the results you can tweak the settings as described in the sections above, or just choose a slightly larger file size and try again.
Saving the image Once you're satisfied with your optimized image, click OK to save it to disk. The file saved will be a copy of your original image, unless you specifically overwrite the original with the optimized file.
