Hey, first post here, and it is because of this thread. What I have found works best is a simple YES/NO link on the first page with the picture, leading to a second page with a fake "600 pics and counting in our gallery", and a message saying "Important, you must enter an email address to get gallery access" and for gallery access, click the "Gallery Access" link, this leads to a redirect on the site, and sends them to an email submit.
I have my max bid on adwords set to 6 cents, and currently run 6 different girls, 5 of them being profitable for sure. With this, I pulled in $14 today while spending $9. I found a good $2 email submit and use that. I had around 260 clicks and around 100 going all the way through to the submit page, of those 7 signed up.
The method Im testing now, which has worked the past couple nights, is turning on ad scheduling in AdWords, and only setting the ads to run late at night, it seems thats when your target audience is online and ready to look at naked girls.