What can I use to adhere paper photos to a wall?
Depends if you want it to be permanent or temporary. It also depends on whether your wall is smooth or textured. If you have textured walls a paper photo will look bumpy and if your texturing material has 'points' it could tear the paper. To avoid that you would have to mount them to a backing of some sort and then they would stand out from the wall a little bit, sort of like a painting. If your walls are smooth you can adhere them directly to the wall.
If you want to replace the photos after a while you can use doublesided tape on a smooth wall fairly successfully. If you want them to be permanent you can use any of the decoupage glues on the market. You might also want to seal the photos before you mount them. There are a number of ways to do this.
Hope this helps,