Obviously you can choose which ever animal (or message) your prefer to appear on the bottom of your friend's drinking vessel.
In this example, we chose a pig's snout.
Step 1: Sourcing and Sticking
Once you are happy that your image is the right size, print it out on photo paper.
Laminate and cut to size
Using double-sided adhesive tape, stick it on the bottom of a mug, beer bottle or drinking vessel of your victim
(assuming your victim is right-handed, the handle should be on the left hand side of the vessel when upturned).
Step 2: Give It to Your Victim
They will look silly.
You can have a laugh at their expense.
Whether you reveal all is up to you.