Click on uploads. Click bottom right on My Uploads. Find the pic, click on delete.
Are using the detailed instruction by Miss_Jayne? Don't forget the right click and save step.
https://www.backyardchickens.com/forum/viewtopic.php?id=118987
You'll have to delete the one in your uploads first. The instructions in the FAQs do work, you just can't duplicate pics in the Uploads.
Bring on that avatar!