Best recommendation on the banner IMO would be to put together a logo shot of the biggest bands you currently showcase. If you look around the various forums (this one included), you'll find graphics people willing to work for very little money and put together art for you. Another option would be art featuring images of music creation (instruments, DAW images of people editing the files, etc.). Or you could go the route of a montage of band member images, but this is less valuable when the bands aren't known.
For the videos, a still image of the band or their logo would be ideal, with your own logo overlaid via the YouTube branding options (top left, right, bottom, etc.)
I will try this out.