There are several reputable sources online where you can find excellent roman coin images. One of the most comprehensive databases is the British Museum's collection. They have a vast array of coins from different periods of Roman history, all available for download in high resolution. Additionally, websites like the American Numismatic Society and the Online Coins of the Roman Empire (OCRE) project provide extensive collections with detailed descriptions accompanying each coin. These resources should give you plenty to work with for your project.