Part of the issue is that you have people from all over the world here, and a significant part of that world views nudity of any kind, be it video, photo, or in your case, painting, as inappropriate for seeing at work. That's the point of the NSFW tag. The person gets a choice on when or if they will look at what is posted, especially if it comes across their feed unintentionally.

While they could articulate this better, I can and do understand the impulse to comment as they do.

Please do consider the point I've made above. You obviously do not have to do anything different, but, at least considering other people's positions is the start to coming to an understanding.