The diploma, (and a photo if your Sim chooses to go to the graduation ceremony) will show up in your Sim's inventory.
HOWEVER here's a small warning: If you put them on a wall and then save the lot or share it on the gallery, the next owner cannnot delete the image or diploma without cheats since it "doesn't belong to them".
So if you plan on uploading or saving a house after graduation, put the picture and diploma back in your Sim's inventory before doing so.