Let me first address Iceland and Ring-billed. Iceland Gull (presumed Merlin was thinking Thayer's as no Kumlien's will ever be nearly this dark), is ruled out by the dark chocolate brown coloring overall, very dark primaries and elongated shape- Thayer's (and Kumlien's) are compact and more "cute" looking.
Ring-billed will never look like this. First-winter Ring-billed Gulls are tiny and have half pink bills, gray upper mantle, and brownish wings. Check out photos online or in Sibley.
The only two options here are Herring and Lesser Black-backed. Lesser Black-backed looks correct here, as the strongly patterned body, and overall structure look off for American Herring.