If they're custom images and not Live Earth/Google Maps/Yahoo Maps as you're saying, then you'll pretty much need to come up with your own way of adding a pushpin on top. The 'pushpin' is nothing but an image sitting above the 'background' image (city blocks).

It's occurred to me that with the profusion of drag-able maps with indicators on it, someone would have come up with some sort of a framework or base-application for this kind of stuff, involving layers of images and other images which can be added on top. Hmm...