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!)
"Engineering in Ancient Rome"
A focus on Roman infrastructure, with many more links. (Thanks Ellie!)
"The History of Baths"
An overview of bath history, including Greek and Roman bathhouses. (Thanks Melanie!)