Hey Powerlifter - this is Dating 101 my friend - everyone from D'Angelo downwards talks to this one.
NEVER bring up sex yourself in this kind of context - you should be telling her the picture makes her look like her Grandma! When she responds (although I doubt this one ever would have done so), "well if you don't like that one, I have far better shots!", you respond "I don't believe you". And you wait for the pics to arrive and escalate from there. If you don't want to play ****y/funny for whatever reason, cool, but just don't make sexual comments to near-strangers - it just comes off creepy.
Mature women are the ones to bring up sex by email/SMS, when their emotionally hooked and want to discuss it. Starting the talk yourself is a slippery slope to either offending her (as here), rushing her, or at worst emotionally cornering her -and when women feel cornered, they usually head straight for the door.
Sex should be mentioned LAST, and not by you. If you want to have those type of conversations with strangers, get on F@ckbook, IMO.