Instagram has updated its community guidelines with specific details on the types of nudity that are acceptable.
The short form of Instagram's updated community guidelines are not terribly specific:
We want Instagram to continue to be an authentic and safe place for inspiration and expression. Help us foster this community. Post only your own photos and videos and always follow the law. Respect everyone on Instagram, don’t spam people or post nudity.
But in a longer form explanation, Instagram gives some specific details about what "nudity" that is or isn't allowed should be:
We know that there are times when people might want to share nude images that are artistic or creative in nature, but for a variety of reasons, we don’t allow nudity on Instagram. This includes photos, videos, and some digitally-created content that show sexual intercourse, genitals, and close-ups of fully-nude buttocks. It also includes some photos of female nipples, but photos of post-mastectomy scarring and women actively breastfeeding are allowed. Nudity in photos of paintings and sculptures is OK, too.
Instagram is also reserving the right to remove instances of naked children:
People like to share photos or videos of their children. For safety reasons, there are times when we may remove images that show nude or partially-nude children. Even when this content is shared with good intentions, it could be used by others in unanticipated ways.
There's also some leeway when it comes to images around hate speech, but only if you're challenging it or raising awareness:
It's never OK to encourage violence or attack anyone based on their race, ethnicity, national origin, sex, gender, gender identity, sexual orientation, religious affiliation, disabilities, or diseases. When hate speech is being shared to challenge it or to raise awareness, we may allow it. In those instances, we ask that you express your intent clearly.