Arachne 
Arachne is an online image hosting site run by the German Archaeological Institute (DAI). It has thousands of photographs and scans from old and out-of-print books, including hundreds of images and an exhibit on the Column of Trajan. It is also useful for exploring the Ara Pacis. You will have to create a login to gain full access but the process is free and easy. Click here to begin.
Wikimedia Commons
Images in the common domain are free to use. Wikimedia hosts the entire collection of Conrad Cichorius' plaster casts of the column of Trajan. You may also discover alternative images of the Ara Pacis, column of Marcus Aurelius or Ara Pacis.
University of St Andrews' Trajan Column Project 
The Trajan's Column Project at St Andrews provides more images and background information on the column. This is a good place to start getting a sense of the history behind the relief sculpture. If you're struggling to identify a figure or item, this chart breaks down the entire column with labels and descriptions. For an extensive list of modern scholarship and other sources for the column, explore Roger B. Ulrich's site from the University of Dartmouth. Another excellent breakdown (in line drawings) of the entire column can be found at the McMaster Trajan Project.
Reed College's Ara Pacis Augustae 
A team from Reed College has assembled an extensive store of images of the Ara Pacis. This site is an invaluable resource for drawings, photos and especially colored reconstructions of what the relief sculpture once might have looked like. Almost every source for images on the monument is collected here.
The Column of Marcus Aurelius
Unfortunately, although resources for the column of Trajan abound, comparatively little has been done with the older column of Marcus Aurelius. One source is the German Eugen Petersen's collection of photography, dated to 1896. One good place to begin deciphering the meaning behind the sculpture is Felix Pirson's "Style and Message on the Column of Marcus Aurelius."
Other Links and Monuments
- Interactive exhibit on Column of Trajan including color reconstructions, by National Geographic
- Virtual tour of the Ara Pacis and the museum surrounding it
- Whitman College project on Greek and Roman theaters throughout the Roman Empire
- Theater of Pompey reconstruction by King's College London
- Digital model of ancient Rome by University of Virginia's Bernie Frischer, including the Campus Martius
- Reconstruction of the Basilica of Maxentius from the University of Virginia's Rome Reborn project
- Mercati di Traiano virtual tour of the ruins of a market built under Trajan
- Reconstruction of Hadrian's Villa by Bernie Frischer, this time for Indiana University