There’s an iTunes (or more accurately quicktime) FLAC codec plugin though I’m not sure if it supports the “PICTURE” metadata and I don’t have a tag editor that supports FLAC to test it.
I just tried the plugin but it doesn’t work. The release notes say:
“FLAC – FLAC decoder and importer for Ogg FLAC (no support for native FLAC file format yet);”
Oh well, I guess we’ll get there eventually.