First of all, here are some good general sites. Includes sections on Greek, Roman, Etruscan, Egyptian, and many other cultures' mythology. Very well done. Easy to use, with images where possible. Covers even minor figures of mythology, with, for example, nearly 600 articles on Greece alone.
Most images are off-site, but labeled and organized.
Thorough and easy to use. A helpful and organized but not overwhelming catalog of images with descriptive text. Includes periods from early history through feudal times. Maintained by Haines Brown. Includes mostly Greek and Roman literature, from Aeschines to Xenophon.