Unfortunately (as far as i know) you can't set the permissions of an included file or image from the page it's in - would be nice though!
So you'll have to recategorise the image as well as the page. Which version of Tiki are you using? There's a fix in 22.x which we're planning to backport into 21.x that properly sets the permissions of a file from the category of the gallery it's in, so you could just move the image to a "public" filegal.
Another thing you might like to look into is the Category Transitions feature?