Ultrasound Photos

Is there any safe way to quickly flatten the printed pictures from ultrasounds? I kept mine flat inside the baby book until I got a scrapbook for them all and they just curl when laid out :(

I thought maybe putting them between two really heavy things that are a little warm but I know they will melt easily just being near something warm (one got a little too close to a scentsy warmer and it caused it to "melt") I'm not really sure on a safe effective way to do it other than gluing them down to pages and I'd rather not do that just yet