Imagemagick app
Imagemagick can do a lot of cool stuff to transform images (e.g. dithering), but the workflow of iterating on the parameters is pretty terrible because you have to manually open the file to preview it every time, change the filename if you want to keep it, etc.
What I'm imagining is an app where you can pick an input image, and it's displayed in the app. Then you can enter the command (could be a GUI, but to start with an entry where you can enter a regular Imagemagick command would be fine), and get a spinner, after which it shows the transformed image next to the original. Ideally there'd also be some kind of history, so you can just re-run the command a bunch of times and then choose the best result from the history.