Several times when I've been texting with girls online that I haven't met yet, they ask for pics. They ask this even though they obviously know what I look like from my profile, which has 4-5 good pics. My other pics are either older, I don't look as good, not showing as high of value, etc because obviously I picked the best for online. A couple of times when I've texted those not as good pics, the texts have just sort of trailed off.

WTF is the motivation of chicks asking for pics when they've already seen some, and how can I avoid missing out due to whatever BS they have in their heads?

It's not a shit test. It's just that they want to see more to make sure you're as they picture you. Just don't bother sending them more pics. Make up any excuse you want. Or say something like "just meet me in person if you're SO unsure." Your long-term solution is to obviously get better pictures.

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Before she can respect you, you need to respect yourself.