I'm pretty sure that we no longer have a 50 post restriction on avatars now.
To get an avatar, first search for pictures using google or something. find one that is already small enough (100X100 pixels) or cut one so it is small enough... Save it in a format that can be viewed on webpages, which is pretty much just jpg or gif.
Now that you've got that done, go to UserCP at the top of the page, then avatar, then scroll to the bottom of the avatar box. Enter in the url for the picture, or if you have it on your computer, upload it. Be sure you are in the avatar section and not the profile picture though.