In the first photo the smaller Common Tern is on the right and the much larger Caspian Tern is on the left. The Caspian Tern is much larger than the Common Tern - 21" vs 12". Also, in the Common Tern the black crown extends down the nape, while in the Caspian Tern it doesn't. In this photo you can't see the bill of the Common Tern.
The Common Tern in the second photo is the single small tern on the right; a large group of Caspian Terns is on the left. In the third photo there are two small Common Terns on the left, the rest of the Terns are Caspian Terns.
Caspian Tern's legs are always black, while Common Tern's legs are reddish in fresh juveniles and breeding adults, but black in nonbreeding adults and 1st year birds.