Ultimate shoulder rides

Told that I may not because the picture was taller and/or wider than 80X80 pixels. It is my understanding that a pixel is a unit of measurement, like a millimeter, of the height or width of a picture on a computer screen.  I have seen avatars on this webite that came from pictures taller and wider than what I tried to upload into my avatar. I am trying to upload picture # 246-21_k2 of video # 246-21 of Getty Images apparently videod by David Russey of Ghislain and Marie where  the man is running and singing with a woman on his shoulders who is laughing with joy.

Since the pictures is less millimeters in width and height than the originals of many pictures I have seen that were also uploaded into avatars, is there a way I can decrease the pixels in width and in height of the one I wish to upload?