Yellow-legged Gull

First picture is a first-year juvenile. Second picture is a 2nd or 3rd year juvenile. The others are, I think, all adults.
All young gulls are very difficult to ID correctly so many of these I first post to www.birdforum.net to get help. Even the experts admit that they can sometimes be wrong. Hopefully over time I will build up a collection of identified pictures to put together as a guide.