When I begin a project, I like to gather images that reflect the inspiration of the idea. The juxtaposition of images start to create new connections and tell an entirely different story when brought together. Some call these collages mood boards, some are content with the collage as the final product.
This app would most likely be for the iPad only as you need the space to move images around, crop, mask, rotate, etc.
There was an app call Mixel that unfortunately shut down their iPad app and continued only on the iPhone after being bought by Etsy.
The early versions of the app would be simple and minimalistic. Rough masking can be done on each image as well as scale, rotate, and moving up and down the layers of the canvas. Simple and clean text tools will be included to help communicate the finer details of the concepts. Images can be imported from various sources starting with the iPad's Camera Roll and hopefully adding support for Dropbox, Google images, and other image sources.
Differing from the old app Mixel, Juxta would lack the social sharing aspect. At it's heart, Juxta is an idea development tool for visual communicators. The ability to email, save a flat file to the Camera Roll, and eventually save to Dropbox would be the most likely ways of sharing the content.