How can I add photos to a canvas frame?

use a hot glue gun to adhere the back of the photos to the canvas. do the same for the photo edges so you that you get a sealed adhesion and the photo wont curl. paint over the edges with a colour of your choice or to match the canvas, so that the photo edges blend in or stand out like miniframes on the canvas itself. Hope that helps.

Attach them with Mod Podge

i also attach them with mod Podge, but i place another canvas on top to give it the texture of canvas on the picture itself.

As others have said here, you can decoupage a picture to a canvas. I would suggest making a copy of your picture on regular copy paper rather than damage an original photo. Use a paint brush or sponge to spread an even layer of Modge Pod over canvas. Place the picture on the canvas and then paint a layer of Modge Podge over the picture. Let dry over night. You can add additional layers to seal further. Mod Podge comes in different finishes including flat, semi-glossy and glossy.