Yep. But unfortunately, the sprites are set on viewport, and since the character sprites viewport is lower than the picture viewport, always on top wouldn't do anything, just place the character sprite on the highest z on their viewport. The picture sprites' viewport is always higher.
What you are going to have to do is make an object class, like Sprite_X, add an array in Spriteset_Map just like the picture_sprites array, then work with that, or a simpler adding of an instance somewhere like Scene_Map and doing that. Your only other option is to display something like 2 pictures, and just change the bitmap on one of the pictures.
I will work something out for you soon.