Diptic is my go to app for photo collages. It comes with a wide range of pre-defined layouts (well over 100) which can all be adjusted. Images can be edited within the app and internal and external corners can be rounded by any amount you set. Frames aren’t just straight edged, collages such as ‘Ying Yang’ style can be created (that is just one example). Borders between frames can be removed and with imagination Diptic can be used to combine images in a seemless way without the look of a collage.