Remove guarantees we do not intend to hold

The fact that we are using gdk-pixbuf for loading files currrently does not mean we will use it going forward. Also, "anything gdk-pixbuf can load" does not mean anything, because what gdk-pixbuf can load is a compile-time option.

As new_from_resource() will assert() if it cannot load a resource, we must be very sure that people do not use anything but PNG and JPEG for resources and the docs were not clear on that.