Photoshop - Optimizing graphics for the Web
Photoshop's Save for Web feature barely scratches the surface of what the program provides. Here we look into some of those mystical terms...
When creating images for inclusion on a Website, many people create the image first in Photoshop and then switch to Animate (WINDOW...Animate) to unleash the full capabilities which basically amount to a trade-off between image quality and file size (and therefore download speed). The only downside to using the two programs in tandem is the sheer amount of processing power and memory your computer needs.
Flat and graded colors: For hybrid images that contain both flat and graded colors, choose the GIF format using the Perceptual, Selective or Adaptive palettes. Remember too that you will probably use GIF (or PNG) formats if you want to include animation in your images or transparency (i.e. a round button on a square, yet transparent background.) JPEGs support neither, so if you created aforementioned round button on a transparent background, in JPEG format the background would appear as white.
When creating graphics for the Web, it's advisable to work within the Web-safe palette as often as possible - these colors will appear the same regardless of platform or browser the site is being viewed on. Clicking on the Foreground or Background Color swatch within the toolbar brings up the Color Picker (right). Make sure the "Only Web Colors" box is checked. Here you can also enter RGB or Hexadecimal colors.