Here are instructions for using water slip decal paper. She is using laser paper - but I use ink jet - I don't have a laser printer. With inkjet decal paper - you need to spray several coats of clear sealer (krylon or any clear spray paint) to waterproof the decal after printing let the ink dry for a bit first).

Also - Bel Decal is a good place to buy the inkjet decal paper from.

Here is a link that might be useful: Water slip decal on soap from Beneath the Willow Tree Newsletter

CB, I copied these instructions several years ago but have never tried it:
Decoupage Soap Bar
Materials
Bar of soap
Modge Podge
Stickers
Instructions
Take any bar of soap and sponge a 2 layers of Mod-podge along the top of the soap.
Let dry after each layer.
Now place stickers or cut out pieces of holiday gift wrap
on top of the soap and again add a few more layers of
Mod-podge.
Your soap will melt as you bathe but the designs will remain.

We did one year for Christmas. We placed whatever picture on top of the soap and painted over the picture with candle wax. No matter how small the soap gets, the picture stays put.