The problem is being tracked here: https://www.ventismedia.com/mantis/view.php?id=18180

Latest build 5.0.2.2503 and no, drag and drop doesn't work.

This is know for Chrome due to the way it handles images, but it works with Firefox. Which Build (Help > About) of MediaMonkey are you using?

Can you Drag & Drop Artwork onto the Properties > Artwork tab?

Pasting artwork in the file properties box doesn't seem to be working. Whether I find an image online and copy it to the clipboard, or use an image editor like GIMP and copy it to the clipboard from there, the 'paste' option is always greyed out in MM. I can save it to my hard drive and then browse to it but I know I've used paste before and it worked fine.