If your phtos are printed on one of the pretreated fabrics that you run through the printer then remove the paper backing I would be curious why you couldn't just ues the photo fabric in the center of the block and add the white around it ( as sashing) and then trim the block to size. If you are sure you want to applique on top of the background, You could use a fusable web but I would definetly do a practice sample first. I am not sure how the printer ink will respond to the heat needed to use the fusable web.