IPhone music album artwork gone?

I plugged my iPhone 4 into my laptop and went to iTunes to put music from my laptop to my phone. Not all of it is missing but most of it that I bought from iTunes is gone. The music that I transferred had album artwork. How can I get the album artwork back onto my phone?

Do you always sync with the laptop?
Did you sign in to your account on iTunes?
You can download your iTunes purchases again from the store.
Go to store, sign in, go to purchases,
Click on the cloud icon to download your purchases.