To pair with the ‘cultural unit’ I presented from the later pages of Allen and Greenough, I’ve done some fishing and found two great online galleries of ancient Roman coins.
First, my favorite, the Roman Coins Database, has everything arranged in chronological order, and small screen captures of ever coin, which lets you click on whatever interests you to get a closer look. A lot of these online galleries have a text box with a link to the corresponding picture, which I wouldn’t consider a ‘gallery’ of very low…currency.
Next, there’s the Roman Numismatic Gallery, which has all the perks of the site above, it’s just a little less forbidding at first glance. It also has some links to other things (Greek coins, Roman military artifacts, etc.) which you might find interesting.