https://www.redblobgames.com/x/1842-...oronoi-sphere/ might be of interest. Using a raster for generation and editing isn't a great idea because you either need to do some shifty things with a connectivity map or waste a lot of CPU cycles working on areas of low information density like ocean basins. Using a sparse set of points distributed over the sphere can be a better option (especially in terms of total points managed), but implementing the operations can be more complex than the basic image processing operations used in tools like Wilbur.

I've mentioned this once in a while, but a relatively simple raster storage scheme should fairly amenable to most image processing operations: store the raster as a square rotated 45 degrees left. The north pole is at 0,0, the south pole is at size,size, and the equator runs diagonally from 0,size to size,0. This gives you something very broadly similar to a sinusoidal projection in terms of the area covered by each pixel, but the connectivity around the edges is a little annoying (e.g. moving "left" along constant latitude from pixel 1,0 gets you to pixel 0,1). This scheme makes painting the poles easy, but each pixel in the map is a varying and funny-shaped polygon (area matters for some algorithms). The mappings from the image domain to lat,lon and back are simple linear ones and I leave those as an exercise for the reader.