Research conducted by Avast has found that the factory reset button on your phone doesn't actually delete everything from it.
The study took 20 secondhand phones bought on eBay and recovered over 40,000 photos – and they weren't all family friendly. One phone contained "more than 250 selfies of what appear to be the previous owner’s manhood".
A Google spokesperson has said that the researchers must have been using older Android versions as their findings did not “reflect the security protections in Android versions that are used by 85% of users.”
This means if your Android is running version 4.0 or later, then you should be safe enough, though Google recommend enabling encryption on their smartphones (go to Settings and then Security).
iPhone users have encryption built in, so they can snap away to their hearts content – just be careful what you do with those photos afterwards!