The rules of what is and is not acceptable to post vary between social media sites, but certain things like nudity are considered universally prohibited. As a general rule, as long as pictures aren't sexually provocative, they are fine to post on social media.
The line between art, exhibition and titillation is blurry and often left up to whoever is vetting the images, so use discretion when posting pictures of full body paintings. When in doubt, err on the side of caution and don't post potentially risqué images. Doing so could lead to getting banned from social networking sites or facing legal action.