The German website Tuxpi offers a simple and easy to use online image editor with 60 tools.
Upload a photo and, for example, swap colors (change color channels) or apply other color filters. Moreover, use all the usual photo editing tools, such as cropping, adding borders or making vignettes. The more artistic effects include adding brushstrokes, swirling colors, and creating various montages and collages.
Unfortunately, the edited photos lose resolution.
Here’s an example of an image with swapped colors (original followed by adapted image):