It's not the website it's you. Most website have file size restrictions for one thing. Another is that if you didn't re size it, your pictures would show up gigantic.
Go to Imageshack.com. By far the easiest pic file image size changer for you to use.
On the bottom, re size your picture you want to use as an avatar to 320 x 240 on the drop down. Then save it. Then go back to your profile and put it in as your avatar. It should be fine then.