Hi Adnan,
[...] This function is actually a menu callback for the "image" path. So any path like "http://foo.com/image", "http://foo.com/image/foo/bar", etc. should trigger this function. But you are seeing the title everywhere. So it could be that some other function is calling image_gallery_page() as well. You can debug around both the drupal_set_title() and image_gallery_page() to see who is really calling them.
Thanks for this pointer; drupal_set_title() might explain what is going on behind the scenes, indeed.
However, as it seems, this has nothing to do with the theming layer and would require serious debugging skills which are far beyond my capabilities, so there's not much I can do against it.
Thanks again for the pointer! At least I wasn't attacked by black magic ;)
Greetings, -asb