How do I manually import cover art into iTunes?

I’ve got about 150 CDs ripped onto my computer. I had no problem importing them into iTunes, but it couldn’t find the cover art on line for some rather well-known artists like The Beatles and Frank Zappa. In WMP, I sometimes have to find cover art and manually import it when it can’t find it online. Does iTunes have a similar option? If it does, it is not evident.