Artwork Keeps Reverting

Ok so I have some track or another I I want to change the artwork for whatever reason so I get the artwork go to 'Get Info' delete the current artwork & replace it with the artwork I want. Job done! Except it's not!! It seems to be independant of whether I have used 'Update Itunes Match' but the artwork reverts to original artwork. I have changed 20 odd sets of artwork only to go back later to find it has reverted back!!! HELP!!!! The only thing I have done which seeems to stop this is if I make an AAC version?