Phototastic
comes with a bunch of ready-made templates that let you create really nice photo collages – and also embellish them with a set of cool filters and frames.
The interface is very well thought out and allows you to adjust the image size with simple finger manipulation. You can make the images look like old passport photos or optimize them for the lock screen.
Publishing on Facebook is built-in, but unfortunately not Instagram.
Download for Windows Phone (Free)