There are many other apps for the iphone, not all wonderful but some are useful and fun, some can give you inspiration to look at photos and your world in a speedy new way without much fuss of darkroom or expensive digital programs. Often a new effect will be just the thing for your journal/ artwork.
This negative effect is done with the Mobits Cam. This app is like photobooth on the Mac, giving you 8 different effects to choose from in real time. That is you shoot what you see in the viewfinder More details here . I thought I would just snap away with it and viewing them altogether I liked the ghostly subtle colours and patterns. Quite abstract, they look good blown up. Available here
a doubt is like a rubber, it can erase the truth
With the Mobits Cam app you also have greyscale, sepia, fisheye, normal, mirror, borders, threshold, and xray. It has a Big button where you can use the whole screen to take a pic, useful if you want to take a photo of you. All in all a useful addition to get your photos to another dimension.