Readers won't have the same viewing experience you do
It is a good idea to see your graphics under different. viewing situtations, to get a sense of the variations your readers are likely to encounter.
First, try viewing your pages under a 256 color monitor. Although increasing numbers of readers have true-color monitors, this is one place where you will still find some variation.
To simulate a basic color monitor, change your monitor's color setting to 256 colors. On a Macintosh, use the "Monitors and Sounds" control panel:
On a PC use the Setting/Control Panel/Display option, then select the Settings tab/256 color option:
If you look at your images in 256 color mode, you'll get a better sense of what your readers are seeing.
Next, try viewing your page under both Netscape Navigator and Internet Explorer. There is a fairly even mix of these browsers in use, and each has some subtle variations.
Finally, if you have access to both a Macintosh and a PC, try viewing your page on the different platforms. You will see system-to-system differences in rendering.