Welcome, Itsadriver !!
Go to User CP and then find the link on the left side to Edit Avatar. Put the avatar you want to use on the Desktop or know which folder you have it in. Go down the Avatar box to the bottom where is says to upload your own image. Click on Browse to find it. Make sure your avatar meets the requirements stated below. Click on your avatar you want to upload and that should do it. Make sure you have the dot in the bullet for Use Custom Avatar. Let me know if that does not work for you.
Note: The maximum size of your custom image is 100 by 100 pixels or 39.1 KB (whichever is smaller).