Hi Verina and welcome. :)
You need to save a picture in a typical format (a .jpg is fine) with maximum measurements of 200px by 150px (with the height being the larger of the two measurements). If it's too tall or wide, then it will come up with an error message when you go to upload it.
The only rules I know of with avatars is no nudity below the waist and if someone is pictured, they have to be legal adults. If you were ever to use a picture of yourself, you can't have other people included in the picture. That's normally a rule for the Elluiki (the wiki, which includes pictures of members) but I think that rule applies to everywhere on the site. I suspect you might not be able to add an image until you're approved as a member. I don't remember seeing those people awaiting approval with portraits.