How will modpodge react if put directly on top of a portrait?

A photo or oil painted portrait?

The photo will react badly. Oil base will let it sit on top. Modpoge is basically waterbased glue and water. What are you trying to do?

You could make a copy of your photo on your printer or at a printshop and then modge podge over that rather than the actual photo.

Hello Marilyn,

I have modgepodged photos, painted wood ornaments and mosaic paper art. Modge podge gives a almost clear but slightly milky thick coating. Its very similar to what white glue would do. I would not recommend it for fine art but for more craft type projects.

Photo paper is a bit thick. A copy would be better to avoid bulges and wrinkles on the finished results.