It baffles me that there are so many image editing apps are on the App Store but only a handful of them does a simple task of image – if you can find the app.
So here’s a dirty hack if you need to create an inverted (negative) image:
* Load an image to any app that has a curve adjustment feature (RGB/CMYK mode), like [PhotoForge][1] does.
* Drag the lower left corner part of the line (shadow) up to the top left. And the top right to the bottom right corner.
* Behold negative!
[1]: http://photoforge2.com/
If you even remotely curious why would one need that in the first place? I prefer darker-colored images for my wallpaper, where such a quick hack can easily turn, say, a white canvas fabric into a nice black one.