Live music. Photos of musicians and singers doing their thing. It could be a public performance, a church choir, a marching band, a street performer or someone just fooling around on a piano.
Here are the rules:
1. No album or CD covers.
2. No nudity
3. No horrific images.
4. Live performance only.
5. If possible, name the band, group or performer and the venue, in the title or the description, not just in tags.
6. The usual maximum upload of 3 per day.
Note: There are plenty of other music sites on flickr, but the ones I saw were either very small or they allowed scans of covers.