This is a picture, not a movie, although you see the image moving. The format is so-called Movie GIF.
I do not know much about the way on how to compile images into an animation, however it is no wonder that Movie GIF is useful when you create a website because of its simplicity.
It is a little bit tough to handle movie files, as compared with image files, on the Internet. Program codes can get more complicated because computers clearly differentiate a image file from a movie file.
By the way, this icon of three people with their arms around each other's shoulders reminds me of my three children, who are really getting along well so far. I hope they help out each other even after they grow up.