First make sure it's size is less then 100x100 pixels. The only way to get the big avatar is be a site contributor.
If the sizing is correct you should be able to simply go into "User CP", click "Edit Avatar" on the left of the screen, click the "Browse" button and find the picture wherever it is on your computer. Click Ok when you have selected it then select Save Changes.
If the avatar is too big, I think it gives you and error message saying it's too big or something like that.
Like Silver said, if you aren't sure on how re-size the pictures post it up and we can resize it for you.
1999 Ranger XLT -- "Project Unknown"
2010 Subaru Impreza -- "The Daily"