Assuming your priming the canvas with an acrylic gesso
as most artists do nowadays, simply use a clear acrylic
varnish (or decoupage glue -- the same thing) as the glue.
If by some slim chance you've used rabbit skin glue for
your ground, use some more of it for gluing the photo.
--
============================================================
For a unique art experience visit:
http://www.zianet.com/jaxart/index.html
============================================================