I was thinking more from a programming point of view. I prefer that the user doesn't have to deal with this pre-creation code directly. I think it would be better that he modifies the fields values in the instance properties.
I did some tests. In the creation code of an instance, i tried the following properties, with different values :
image_angle (already reported by BreakDown)
You forgot about image_blend but ince I don't use that in the room editor personally I could care less if that's added. As for scaling, that would be amazing. I look forward to using it (if it's ever added)