I'd not consider a full body pic of a nude guy a dick pic. Obviously a close-up would (erect or not) as would a torso-only shot from the front.
This has come up before for women as well. While some women and men can certainly pull off a sexy completely nude shot, I personally think that pictures that show a tease of what's underneath can be even sexier than a full nude shot.
I can see some women perhaps wanting to see a dick pic at some point if size is an issue for her (either too big or too small), but the Mrs isn't one of them. She'd rather see a pic of sharp dressed man than a naked one.
If the dick pic is from a bi guy, then that's a bit different. As far as I can tell, the group that most enjoys seeing unsolicited dick pics are other bi guys. In that case, it is purposefully meant to be a dick pic. I think that it's safe to say that more dick pics than not are bi guys looking for other bi guys, even if the pic poster claims to be straight.