You should be able to visit this link while logged in to "My ebay" and then you should be able to add / edit your store logo :]

http://storefront.ebay.com.au/psn/edit

Alternatively, you should be able to go to your profile:

http://www.ebay.com.au/usr/6508.froggy 

click "Edit Profile" and then hover your mouse over the profile picture and a little edit icon should pop up in the right hand corner of the space where the image goes. You can click that to edit it.

Hope one of those options works! :]