Member Image Icon

Ok, I have a stupid question

I am adding new membership types and to create a new membership type, I am 'required' to upload an image icon. Can anyone give me some detail on this? What size are they? where are they used?

The icon is to show on your My Memberships page, the picture is displayed next to the Purchase and Info parts of the membership.  The image is about 115x115 in size max.  if you do smaller it just centers in the square.

Thank you ...

I'd like to change membership level icon images - could someone point me to the file location please?