That's pretty easy, just like in Sims 3. Clone the picture with Sims 4 studio and edit the picture. Easier in fact as you can use Microsoft Paint instead of GIMP.
Today I modded some shelves in Sims 4 studio so I could put crystals and metals on them. I hated the mess that the shelves were in - each shelf was different. Now I have a rack with 9 large spots, another rack with 12 medium spots, and a third rack with 15 small spots! Great for serums, metals and Crystals.
What has me confused is XML mods. Easy in sims 3, a big "WTF are these XML tags?" in Sims 4! All I want to do is make it so that the "Need Fixer" serum doesn't fail half the time.
Well, I also want to mod either the SimRay to change genders of a target sim or the Object cloner to have an option to clone a sim but change genders. I'm surprised that hasn't been done yet.
Sims are better than us.