I'm a noob. How do you change the main picture of your listing? 

F72a44f3 61f0 477c bca5 2844f6e81ab2glitrz
I don’t have any active sales but, from memory:
Click on your user account,
Click on "selling" (you should see all items you have for sale),
Click on "actions" (on right side of screen),
drop down will show you stuff like "edit listing" and "edit photos",
If you don’t see edit photos, click on "edit listing" and you might find it there... Good luck!
You can always ask Audiogon.