Can you take a regular photo and Mod Podge it to Plaster?
I'll start by asking are you 100% sure it's plaster? I kind of looks like paper clay to me, but that could be the effect of the photo.

If it is plaster:

Plaster likes to absorb all the water it can - from what I remember Modge Podge is rather watery in comparison to other glues. But I believe it can be used on plaster as it is basically a type of PVA. 

It'll stick to the plaster, but will sit on top of the surface like a varnish. Moisture will be absorbed into the plaster, the plastic part of the glue will remain on top. Make sure your photo isn't going to bleed when it gets damp (Do NOT use inkjet photos). Those colors will likely sink into the plaster if they run and your photo will be ruined. If you want to protect the whole piece from absorbing finger grease and various other stains that tend to make their way to ornaments despite it being kept in a clean place - you could coat the plaster in it entirely using it as a sealant.

Just double checked the MP website. They say before using it on plaster, wipe plaster down with a damp cloth - likely to remove any dust - I wouldn't go too damp however. If you saturate your piece too much it might become delicate depending on plaster type.