Ok, here is how it breaks down...
Create your avatar: 170x240
Download the blank .gif image, 170x240, to use as your avatar.
It must be paired with your background image.
Create your background: 180x500
Upload all images to ATS by clicking Tools+, ATS Uploads
Now, click on the image once it's uploaded to ATS and choose either Avatar or Background link. You can double check that it linked correctly by
viewing your profile and in the profile, the corresponding Avatar and Background areas. You should see the url information listed.
Remember, if you wish to use a Background image, you must use the blank .gif image in the Avatar slot. You can come by this image by right-clicking
and saving to your computer from any member that is using it currently.
Example: SO is using a Background image with the .gif as the Avatar. HERE
right-click on the image and save it to your computer. It will be the .gif image that is saved (not the background image)...