When iTunes can not find the artwork, you can add it manually.
What I do is open both a web browser and iTunes (on the computer).
Do an image search in Google to find some artwork that you like.
In iTunes select all the songs in the album. I do this by sorting on the album column, clicking on the first song, then shift clicking on the last song.
If you have the little album art window open at the bottom left of iTunes, you can just drag the art from the browser window to the art window in iTunes.
Or, you can right click on all the selected songs and choose Get Info. It will warn you that changes will be made to all items. Go to the Artwork tab. Copy the art from the web browser and paste into the art box in iTunes.
I'm on a Mac, so if you're on Windows you may have to adjust these procedures a bit, but it shouldn't be too hard.
Next time you sync your iPad with iTunes, it should add the artwork. Sometimes it takes more than one sync.
