AI pipe the border using a 4 or 5 tip, then begin to messy fill it in, trying not to push the border out. Then I use an angled spatula to pull buttercream off of it, starting from the border, in, still not moving the border. It should look fairly smooth, with spatula marks. When it crusts, use a viva on it lightly. I'm almost betting none of that made sense but I'm hoping it did! I work primarily buttercream so I do logos and stuff with buttercream all the time like that!
I do a lot of pictures or designs with buttercream as I have a projector to get the image on the cake but I don't usually do it in the thickness of this picture. I didn't know if frozen buttercream transfer would work or not.