This is a complex plugin, but if possible, players could link paintings in-game to the image of an external URL on the web. That way, all players on your server could see your paintings. This would also need a approval system, so that the admins can approve a painting image before it displays (To avoid abusive images). If this is possible, I'd be one of the best plugins ever created. Allowing fully custom images/content. Everything including: Banners Advertising Rules/Instructions Previews of areas you wanna visit Lists of commands Pictures on how to do things Diagrams AND MAPS! Without a doubt, the creater of such a plugin would be renounced among server admins as a god. And the social status among the community would be as popular as Essentials, WorldEdit, and Voxel.
There is one I use called MapAPI,it's discontinued but if you download it has /mymap for client amp only and /write for maps for everyone
It is possible, but only with the colours minecraft have. Minecraft doesn't have many colours so it won't be very good quality.