Has any of your friends seen you wife/GF naked?
Only one friend saw her boobies after a top she was wearing in our garden during a BBQ gave full view of her puppies when bending a few times.
Was it on purpose or by accident?
Accident as she's not like that 
Have your friends ever seen naked pics of your wife/GF?
Yes, quite a few, I am a photographer and she has modeled for me
In any of the above, how does your wife/GF feel about it?
The latter, depends on who it is, other photographers fine, learing friends no so!