Here's the bird in flight.
At rest it has noticeably paler primaries than the Herrings (compare it with the the 1st cycle Herring behind it) and a relatively thin bill. Thayer's Gulls on average have less moult from juvenile into first winter plumage, note that the Herring is whiter headed and has a lot more marbled gray feathers in the back because of this process. The tertials are also a paler medium brown than the Herrings and have patterning limited to the edges (as opposed to Kumlien's Iceland which has much more markings internally).
Another view of the Thayer's in flight. Even as juveniles they exhibit the "venetian blind" pattern in the primaries of dark leading edges to the primaries and pale trailing edges. Note the contrasting secondary bar (which is more prominent in some lighting angles than others); Kumlien's Iceland Gull has much less darker coloration in the secondaries (or tail).
Here's a distant comparison of the underwing with a 1st cycle Herring Gull on the left. The Herring has much less white in the underwing (though in bright light can look quite white as well).
Finally here's a tricky exercise, can you find 5 species of gull in this average at best photo? (This is definitely an example of knowing what the photo shows already makes it easier to see).
Our Thayer's is the gull near the middle. A first cycle Herring is in the foreground on the left (and a few more Herrings are scattered through the mass of Ring-billed's). The Parasitic Jaeger looking thing facing left in the water on the left is a 1st cycle Lesser Black-backed (this is probably not ID-able based on this photo alone), and close to the right side of the pic in amongst the Ring-billeds is a larger-billed bird with a spangled back, a 1st cycle Great Black-backed.