Image searching is a great automated testing method for verifying that DRAWING code is working correctly. It's not a good method for pretty much anything else in the GUI space.

For real image searching, there are a lot of interesting tests. Juerd mentioned using the regex engine, which has its merits. There are other sparse comparisons which help find exact pixel matches of various swatches. Another poster mentioned image mosaicing methods like least squared errors, but those are better for situations where the alignment, rotation, scale, coloring of two images are not identical.

Otherwise, please please please dig through the window manager to verify text strings, not pixels. Any change in the test user's Appearance preferences, or operating system version, or even a different video driver, would totally blow all your tests.