If you didn't pay a lot of money for it I'd try the matte finish. The decoupage won't damage the print if it is on regular paper, but you could end up with brush strokes. This might be ok.
If it is on what we used to get photos printed on back before everyone printed their own. If it has that type of emulsion you shouldn't put deoupage glue on it.
If you're not sure get advice from someone who can look at the print. You could try it on a corner first.
Good luck,
Kathy