The main reason why are photos picture framed with a window mat In this moisture-damaged example photograph, moisture or water has caused the water-soluble emulsion (the top part with the actual image) to partially liquefy and adhere or stick to the picture frame glass. This would probably not have happened had the photo been framed with a mat (or window-mat) to keep it away from the glass. Picture frames with photos should always .. be matted or have window mounts which, as well as visually enhancing the photo, help prevent photos from sticking to glass. Glass ...