Sometimes web designers break up a single image into slices of images. They do this for many reasons, but the question is - can you optimize that image for image search?
John from Google chimed in about that in a Google Webmaster Help thread. John basically said, when you have this situation, you want to assign the alt attribute to only of the the images and not all the additional pieces of the image.
Of course, John doesn't recommend slicing up a single image into multiple images for a few reasons.
I'd recommend applying the alt-attributes only to one of the slices in a case like this. In general, I'd probably recommend not slicing images though, unless there are important reasons to do that. For example, it would be much harder to recommend your images in Image Search if we are not able to index the images as individual images.
Regarding the choice of slicing, I would take a good look at whether or not you're actually saving load time. With multiple image files, you're forcing the browser to download more files than would be necessary, with all of the associated overhead (also, most browsers have a limited number of parallel downloads from the same host, so the multiple slices might be blocking more important page elements from being downloaded first). Also, I would double-check to see if you can't achieve the same (or similar) text effects using HTML for the text; Google has a new font API that may be able to help. If you can use HTML text instead of having it in the image, you can skip the question of alt-attributes and make your content more accessible at the same time. These things aren't critical, at any rate, but I thought I'd mention them none the less :)
Have you run into this?
Forum discussion at Google Webmaster Help.