Bowling Green University and Purdue University have some interesting image libraries for algae and cyanobacteria. The Bowling Green site also has a link to a program that was written to aid in counting specimens in images.
Here they are!
Bowling Green (Image Library and other algal links)
Also, I found this book, written in part by an NC State professor, is availabe at NCSU library. Hopefully I'll be able to check it out the next time I go in.