The "Big Picture" answer is to promote your best/most attractive feature, but the problem is a lot of people aren't aware of what their most attractive feature might be.
Lots of people hate that they have, for example, curly hair, dark brown eyes, a lot of body hair, or any number of other qualities that aren't common or trendy at the moment because they polarize opinions and other people tend to either like or dislike them actively instead just saying "meh", yet those are the very features that make them stand out from the crowd and get noticed.
As for dick pics, that's a dangerous spin of the roulette wheel if ever there was one. Yes, it is incredibly shallow to judge a guy solely by the appearance of his dick, but if that's all you've got to go on then you have no other choice. Also, bad lighting and poor photography skills are working against you in the dick pic arena to begin with.
In summation, I'll agree with previous posters that a teasing pic, maybe showing full or near-full body (except head/face, of course) with the dick covered but not entirely obscured by a sheet or blanket, a towel, or underwear is best, and save the actual dick pics for special requests only.