There are several ways to differentiate the Thayer's Gull from the Iceland Gull (since the tertials aren't visible in this photo)...
The juvenile Thayer's Gull has a notable tail band, whereas the Iceland Gull is flush & pale throughout the tail... THGU has contrastive primaries with the rest of the wing, whereas the ICGU is a little more flush and pale throughout the entire wing.

Here's another photo I took of Iceland Gull and Thayer's Gull next to each other
http://www.pbase.com/birdfedr/image/119465446