$2500/mo from AdSense on http://coverphotofinder.com/ Zero-maintenance, which is nice.
You know, what I would love to know is how you attracted the initial audience. This stuff is always so easy to make, but my past attempts never really got enough traffic boost. I'm wondering how you got over the 'plateau' (which in my experience seems to be ~1000 visitors a day.
Sorry, forgot about this thread! We were first-to-market (may have had insider information about the Facebook Timeline launch...), and got on Mashable. Spread through the blogs and hit #1 on Google for "Cover Photos". We've stuck that #1 spot and it's where 80% of our traffic comes from.