It is probably no surprise to anyone that search engine robots have a problem deciphering what an image represents.  That is why there is an ALT attribute you can use to describe the image in terms that search engines can understand.

In this video Matt Cutts takes us on a three minute journey to describe what the ALT attibute is and how it should be used in a way that does not spam. 

It is pretty basic information, but worth watching if you have any questions at all on how search engines interprete graphic representations of pictures, video or graphic arts.