Ouch! I have some that have taken me quite a long time to find. I will make sure they are backed up…and I think adding an option for user preference is ideal!
Please add your support to the feature request linked above.
A poor work round if the original image came from Google…
Turn off internet. Restore from a suitable backup. Choose your artist and share. Deselect all the genres, born, died etc. Internet back on. Share to imgur. Open image and search Google for image.
Not worth it if you’ve got a lot, but if there was one where you’d forgotten where it came from it may be useful.
Eh? 
Or just surrender to the intricate workings of Roon.
One things for sure, I’m not going to bother anymore with personalizing Artist Art until this is resolved.
Just when you think it is safe to start editing Artist pictures, this starts happening again. 
Unfortunately, this is a complex problem requiring a big change in how our metadata systems work. It’s not about “best” – it’s about Roon’s understanding of the artists, and how the tracks and albums in your library get matched to the rich artist data in our system.
I discussed this in more detail here:
The good news is that the work described in that post has been underway for quite a while. When complete, it should ensure a more durable understanding of artists and other content in Roon, without sacrificing rich metadata or edits.
It’s a major architectural building block – not a bug fix for this specific issue – but it should largely resolve the underlying cause of these lost image edits.
I don’t have a specific timeframe yet, but I did want to be clear this isn’t being ignored.
I’m just curious, but what percent of artists have official Roon images? To me, all of the empty image boxes are deeply disturbing. I wish that the powers that be would focus on fixing this rather than supplying us with features that no one has requested