Yale Image Finder
It searches the text within the images, but you can also search on image captions or on the article text. I tried it using the search string streptococcus pneumoniae. Searching the default of "image text (high recall)" found many genome and phylogenetic tree images, and a screenshot of the ISABEL diagnostic program. Searching "image text (high precision)" found the same (I assume) 18 images. Searching captions instead found more, including an image of S. pneumoniae infection. You can search within, say, captions and full text.
There is more detail on how this works in a paper in Bioinformatics.
BioText, from Berkeley, searches image captions and article full text.
Tidying my desk I found a paper from Bioinformatics from 2004 describing FigSearch, a text mining system for figures from full text biological papers, developed at the University of Oslo. But the URL is no longer found, and a search of the UiO site finds nothing. A Google search for FigSearch finds lots of things about it, but not it itself. If you know where it is, please leave me a comment.