Solution for missing Album Art on iPhone after updating to IOS 8.4
Connect you iPhone to your Mac or PC. Go to iTunes on your Mac or PC. In iTunes go to Music and uncheck Synchronize Music then Apply. Wait until it's ready. Then check Synchronize Music and Apply. The library will then sync again and all Album Art will be visible again on your device. It will take some time to set it all back.
iMac, OS X Mavericks (10.9)