Gull ID anyone?

This one has us stumped. It has also stumped some pretty knowledgable folks both here in Texas and in our new Minnesota connection. This photo was taken last Sunday at Goose Island State Park in Rockport, TX. I snapped the pic because the bird was larger than the Laughing Gulls it was hanging out with... and obviously was not one of them.

There had been reports of a young Slaty-backed Gull in the area, and I thought we might have him here. But this photo and the other photos of the Slaty don't quite match up. Any guesses?

NOTE: We're carrying this as an Iceland Gull in the photostream.