I have a lengthy step-by-step way to get back missing art (not sure if it will work for you...it might depend on what kind of mood your iTunes is in). It's a variation/combination of a few previous posts, so many thanks to you! Also, I'm MANUALLY transferring my music to my iPod (takes longer, but it's easier for me), so all this is written from that option. Even when iTunes can detect the proper album art, it's still not transferred.
---For info, i'm using iTunes 10.6.3.25, with an iPod Touch 4G (64gb)---
1) I deleted those offending, artless songs I wanted to replace.
2) Highlight the song(s) in iTunes that you want to add art to. Right-click on them, and select "Create AAC Version". These new files are then paired with the originals, all with the same titles. You can see which is which by right-clicking on the top display bar, under the apple, and select "Kind".
3) Right-click on the AAC song, and select "Get Info", then click the "Artwork" tab (note - the artwork tab is missing from all my mp3 info menus). Also, if you don't/can't use that previous "Kind" option, don't worry - all the info menus also have a "Summary" tab that shows the file sizes - AACs are larger.
3) Click "Add" to select the picture file you wish to use, then click "OK" (I keep all the .jpgs on my drive, in case iTunes can't automatically find them). Note - for me, some of the songs already have the art showing there...ignore that! - I still must manually add the .jpgs (there will then be 2 pictures showing, but that's fine).
4) Drag & drop the AACs to the device, double-check that the artwork appears there, then delete the original mp3s from the iTunes list.
Hope this helps some of you! If not, be sure to check the other posts here.