The song must match several items to get album art from the iTunes Store. Track Name, Artist, Album and Genre. If these don't match then you wont get art or get the wrong art.
If the song does not exist in iTunes Store, there will be no art downloaded.
The Album Art from iTunes Store is corrupt. This can be fixed by closing iTunes, delete the My Music\iTunes\Album Artwork folder, open iTunes (with Internet connectivity) and eventually iTunes will download the art. If you play a track, it may force iTunes to get the artwork from the iTunes Store.
iTunes album art is disabled. In iTunes, Edit->Preferences, Store tab, check Automatically download album artwork, click OK
2. If you don't use iTunes Store to get your album art
- Find the picture you want and save it to your PC, right click the track that you want to change, choose Song Info, click the Artwork tab, Click Add Artwork, find the file you just saved and select it. Click OK.
If you do #2 and you are looking for the results to appear on your iPad or iPhone, you will need to remove the songs from your device via a sync, then sync the tracks back onto the device to see the album art update.