I used to mount photographs for club competitions with Evo stik.
Spread a SMALL amount VERY thinly, using the edge of a bit of cardboard on the photo and also on whatever you're sticking it to.
Let the surfaces dry, then position the photo very carefully in place . Rub lightly with a cloth to get rid of any bubbles.
You get an instant and permanent job - it will still be stuck when the calendar is history!
I used to mount 20"x16" photos this way, but if the poster is just lightweight paper, I wouldn't recommend it. framing it under glass would be better.