Thanks for testing!
I am afraid this is for me a real and consistent issue with a clear starting point (after build .173), but I cannot figure out what causes it and how Matt shall be able to reproduce. I am on Win 7 64-bit running just one instance of MC on my internal network. I have tried pasting different sized image files, with and without split view and in views showing and not showing thumbnails, all with the same result.
After more testing it seems that pasting to a flac file that already has an image embedded with MC (using earlier builds) actually works, but pasting to a flac file that already has an image embedded with MP3Tag causes a crash, just like pasting to a flac file that has no embedded image (crash every time). The image file is created in the music file folder even when MC crashes, so it seems to be the embedding/writing to file tag that is causing the crash.