Sites with mainly pictures:
Maecemas: Images of Ancient Rome
This is a very nice site, full of pictures from all over the Roman Empire. They are organized very cleverly: first by country, and then clicking on an architectural site name takes you to a new page with a list of pictures of that structure. Another great thing about it is that it's set up so that no page takes a long time to load -- so you can easily access exactly what you want in a short amount of time. Also, the pictures themselves are very good; most of the images on my site are from this one, by Leo C. Curran.
Sites with pictures and info:
"Map of Ancient Rome"
A nice visual overview of sites in Ancient Rome. (Thanks Pinewood Elementary 5th grade class!)
"History of Roman Architecture"
A collection of the most famous Roman structures; a list of clickable thumbnails, each of which gives you a larger image with about a paragraph of information.
Sites with more links:
"History of the Rome Colosseum"
Links to a wide range of Colosseum resources. (Thanks Theresa!)
"Roman Colosseum Seating"
An overview and many more links. (Thanks Alex!)
"The History of Baths"
An overview of bath history, including Greek and Roman bathhouses.