First - thanks for a great tool - I've just installed Subsonic and am very impressed.
So here's my feature suggestion - a pretty small thing: It would be nice to have an upload button for cover art instead of just the URL text box. Some of my cover art was either really hard to find the first time (and I don't remember where I finally found it), or I scanned it myself. With an upload button I would export it from iTunes and load it from that machine.