Using Dani's suggestion and the duplicate site workaround, you should still be able to do what you want to (changing image).
When you change the template and click 'save', then yes you should get a message box asling if you want to update the pages based on the template. The template/folders/files relationship should be intact though, - the duplicate site definition is the easiest way to do this, otherwise another Templates folder is created outside of the CMS folder and messes with the updating and my head.
I've lost track of what I'm going on about now.
If the message box to update the pages doesn't appear (and this does seem to happen on and off indiscriminately), then the proper menu item to force updates is Modify > Templates > Update pages.
If your store is online, or set up on your testing server, you can edit the Admin Template by logging in to the Admin area, setecting 'Content' > View/Edit Content > and select 'Admin Template' from the list. This updates the data in the PowerStore database, in table ps3_contents. This is probably the easiest way to do it!
Agree about the scary choices.