For a collage, I'd actually stick more with those clear square adhesives... you know which ones I mean? These strips are a lot longer and probably would be damaging to the photos when you took them down. They're better for a bit sturdier things.
I haven't used them for collages, but I have used them for heavy frames and they stay up. I would recommend maybe trying to put them up without any glass if you have big collage frames and see how they hold that way first, then add the glass in.