We have had this talk fairly often after we set up this site. as @AhabGreybeard said some study sketches and even painting with nudity is allowed but should tagged as nsfw. What isn’t allowed is porn or erotic content. As for what constitutes porn or erotic it depends on person to person and also country to country. So in general other than painting which won’t have the characters in them doing sexual act, doing some sexual poses, showing underage people doing those act etc it should be safe. There is not comprehensive technical list of things that can’t be posted here it is all based on common sense.
There is also some discussion in this thread - Content policy?
So in short it is okay to have nudes in your painting. If the artwork contains nude or images which are not safe for work or children always add #nsfw tag. If you think your work is going on the border of porn or eroticism then do not post it here. Since this site is available in any countries we don’t want to take risk or compromising the future of website.