I think it's been mentioned before, the idea of, instead of many windows for many images, one window with tabs for each image you load. This would make it easy to...say...take a picture from one, switch tabs, and paste it in another. Firefox has tabs, Visual Studio has tabs, Azureus has tabs.
I'm an advocate of 'taskbar conservation', and all those paint.net windows become a pain to navigate when there is more than one.
I like Paint.NEt but I think that if it joined the 'Tabbed Everything' revolution, it'd be awesome. not to mention productive.
Hope it's easy to code.