It's called
PonyPhoto 
Link:
http://www.windowsphone.com/en-US/apps/4c17dffa-eaac-4e88-8ca9-8718d5ec3432Features:
normal function:*Multiple image effects. PonyPhoto comes with various effects such as Invert, Grayscale,Jitter, Water Wave, Gamma Correction,Oil Painting, Rotate Channel, etc.Totally 18 kinds of effects.
*Collage. Various Collages templates are available for compositing different photos to generate a new one; and editing the saved collage image with other features.
*Frames in various styles. Like simple frames, glaring frames, and color frames
*Image color adjustments. It is to adjust with brightness and contrasts on a selected photo.
*Changing image’s shape and size . To rotate or cut a photo to generate a new one.
*Images save and share. To save the edited images to the default directory, or send them to Facebook, Twitter, Flickr .
unique function:Composite photos to a .gif image.PonyPhoto could add several photos into a dynamic image with .gif format, and I didnt find any other app that could do this
