Second Nature (I think).  They have a series of 
screen savers based on different themes (Space 
photos, Flowers, Impressionist painters, etc.), 
but the included art can be disabled and you can 
add your own jpg files.  It's about $10 (I've 
seen it recently at CompUSA or online at 
http://www.secondnaturecd.com/  (where you may be 
able to download the software and sample images 
to try out).  One caveat, my software is about 3-
4 years old so there's no telling whether the 
current software is as flexible as the version I 
have (i.e., will work with jpgs and can handle 
hundreds of images).  I'm sure there are lots of 
free downloadable screen savers that will do the 
same thing.