The Coverart pics have to have the same name as the roms they apply to and the Pics also have to be in .png file format.
Fuzzy_Renamer allows you to choose the folder with all roms for a specific system and chose the folder with the correct coverart pictures and it creates new pictures in a new folder of your choice with the correct name and file format. It doesnt work 100% but it does work and it will save a lot of time.