Firstly you'll need to import all the files to iTunes (so let's hope they all have their tags!). There is a setting in preferences to automatically get album artwork, but if for some reason that doesn't work, you can right click on a track in each album and go "get album artwork". If the album's in iTunes, it'll get it. So lots of Beatles and AC/DC albums and you're out of luck, but most others'll be fine. If you're left with any without artwork, first go to the iTunes store and check that the album isn;t there - if it is, it probably just has a slightly different artist or title in your library - change it to match what it says in the iTunes store and try to get artwork again.
If all else fails, download the artwork from somewhere like cdcovers or amazon, select all the tracks in the album, right click/get info, there's a box that you can drag artwork onto.
Shouldn't take long.