I don't know about for Mac, but Windows usually has a program called Paint that will allow you to resize your picture. Open your picture in paint, click on the Image menu, click attributes, set it for pixels and adjust set it for 120x120 or less. If your pic isn't an exact square (or at least close) it's gonna look funky. Say the original size is 700x600. In this case you would change the 700 to 120 and then try something like 100 or 90 in place of 600.

Slicker image editors, like Paint Shop Pro or Photoshop can auto adjust the second pixel setting to keep your picture looking 'normal' when you change the first setting, taking guess work and calculations out of the picture. (Pardon the pun. )

For animated Avatars you need a gif animator program. A gif animator will take a series of pics and 'play' them one after another. Sequential pics will look like motion (depending on how fast the pics change) while random pics will look like a slideshow.

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these