How to switch album covers on iPhone?

Recently, I synced my iPhone to the iTunes on my computer. Now, a lot of the pictures for the albums have been switched with other album covers from other songs I own. Why did this happen, and how do I change them back? I have tried deleting and re-downloading songs, and that works for some, but others will just switch back.

http://www.wikihow.com/Change-Album-Art-in-iTunes