How much are aircraft nose paintings restricted by regulations?
It used to be that nose art was pretty outlandish and sexually charged, and it remained largely unrestricted as this B-24 from WWII demonstrates.



With the current gender integrated units and women serving in combat squadrons, this kind of stuff is non longer acceptable.  Currently there's a rule, in at least the USAF, that nose art must be 'non-gender specific' i.e. you can't paint naked women on your airplane.