I bought 50cm square canvases and painted the backgrounds with acrylic paint. For this canvas I cropped the photos to a square and then sized them all to 12cm squares to print out on ordinary copy paper. I used Matt Gel Medium to stick the photos to the canvas and also applied this over the top to protect them. I found through trial and error that it was less likely to wrinkle if I applied the gel medium to the back of the photo rather than the canvas.
For my daughter's canvas below I wanted a less formal look so I printed out the photos in rough so that they were no larger than 9cm if they were rectangular and made up my design on a piece of brown paper first. I could then work out if any photos needed to be re-sized to fit better. I made up the canvas in the same way, finishing with a couple of wooden tablets I bought, that had special messages on.