There are a few BeOS image editors that we have the source code for. There has been some recent discussion related to this which you might find helpful, for example this comment of mine from a few weeks ago:
If you are new to the BeOS/Haiku API it might make sense to look at some of these existing programs, either to get ideas of how to write an image editor on Haiku, or maybe to work on one of these and make them better and more modern. These may already be in the more simple style you want.
I link to a few in the above post but there is also Becasso: GitHub - HaikuArchives/Becasso: Paint and imaging software for Haiku, originally written for BeOS by Sum Software.