How do I go about placing a banner ad?

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Okay, I admit it. I'm stupid. I've been clicking links in ptc sites for some time now. My pennies are adding up, but I want to place a banner add and that's where I seems to have walked off a cliff somewhere. Someone please explain how that is done? On one PTC site I have over 7000 points and that's enough to place an ad. HeLp!