Back in middle school we used something called Mod Podge. I think it is still available. I'd do a test first before trying it on a poster.

Sand the wood smooth.....glue the pictures on the wood using a cellulose type of glue.....whan ti dries coat with Modgepodge....if that the look you want....other wise cover with several coats of non yellowing varnish.
Linda c

She did it here with a copy of a photo..

Here is a link that might be useful: http://www.josiejones.com/2012/05/photographic-block-art-image-needed/

Ive decoupaged, but not a poster....I've used mod podge under and over the paper. Only thing to be careful of is once the paper is wet it will stretch so it may rumple if you're not careful.