Going back to gallery key and button

I don't understand how to go back to the gallery view. 

I would love to have the back (mouse button 4 key) to go back to gallery view. Since you already navigate with the back and forward keyboard key you don't really need mouse button 4 to go back a page.

If you don't want to put that in its fine, but can you tell me how to add that so I can have it for me?