The example file has 1 embedded JPG picture and the type is "Artist"
For FLAC files, at least 1 embedded picture must be "Front Cover" type.
Multiple "Front Cover" type pictures can be embedded: Winamp will display one of them, probably the first found.
This is easy to test by using the "Set cover type" function in Mp3tag.
I believe the maximum size for each picture is 16 MB, which is a FLAC limitation.
Embedded PNG pictures can also be displayed.
This is slightly different than the behaviour for MP3 files (
see above), which illustrates
the point made by thinktink.