See also Category:Wikipedia requested photographs of biology.
Easy pickings: a list of articles with corresponding pages or categories on the commons!
These are lists of articles with taxoboxes in an en.wikipedia database dump (from http://download.wikipedia.org), that do not have an image.
In the 2005-12-11 database dump there were: 6930 articles with taxobox, without image.
In the 2006-01-25 database dump there were: 7157 articles with taxobox, without image.
In the 2006-03-26 database dump there were: 8449 articles with taxobox, without image.
In the 2006-05-18 database dump there were: 9924 articles with taxobox, without image.
In the 2006-07-02 database dump there were: 11356 articles with taxobox, without image.
In the 2006-09-25 database dump there were: 13808 articles with taxobox, without image.
In the 2008-01-03 database dump there were: 58010 articles with taxobox, without image.