Apart from possibilities like
a) I don't show my tits unless you pay and this was the only thing to hand
b) I want to prove it's an up to date photo
c) I tried to do a verification photo but couldn't work it out
d) "The authenticity or legitimacy of this member's profile has been brought into question."
e) I'm really stupid with mobile phone and had to balance it on its side to take this picture
f) I'm Romanian, even though I don't look it
g) My pimp says I do bareback though I have an open-front nightie that keeps my back warm
h) I'm a bloke you idiot – the picture is just a requirement before I add some random shite to a PG gallery.
i) I'm a bloke you idiot – the picture is just a requirement before I use the profile as a sockpuppet
j) I'm a bloke you idiot – the picture is just a requirement before I use the profile to set up fake feedbacks
k) (least likely) I've heard of David Bowie and thought it would get your attention. Could you send me a downpayment for an overnight?
l) (fairly unlikely) I'm a con artist but not a complete cunt. All respect to Mr David Bowie [yes you are: otherwise you'd use his real name]
m-z on requets. please send bitcoins for my private gallery as I need to buy a camera.