I have dBPoweramp and Mp3Tag.
Mp3Tag is (to me) slightly more user friendly. It cannot be easier. It is free, but it asks you to donate if you wish. I do about once a year because I do use it often.
In case you decide to use Mp3Tag, remember to delete old picture AND save that deletion if you want to change the picture on the file. Only then add a new one. Do not worry, it is about 1 second of extra work.
For me, WAV covers seem to be more related to the machine it is played on than to the program it was tagged with. Same file will show on some of the machines and not on others. Even from the same brand.
dBPoweramp converts pretty much any format I use. I could not discern the difference between FLAC and others.