A long time back I visited a site that had a very fancy, animated navigation bar. Now, as a professional web developer, I'm not in favor of DHTML-supported, fancy navigation bars, but it was very fascinating. What they had done was, whenever you hovered your cursor over a link, a big, comics-type dialog balloon appeared to give further details of that link. I wondred how they did it, but then it slipped out of my mind.
That technique uses layers and Cascading Style Sheet definitions, and I'm going to tell you here, how it is done. Nothing pyrotechnic, but it'll pay a way to more complex tasks.
First, demo. I believe once you visually see it, you'll understand better what I'm trying to accomplish here. Given below is a link. If you take your cursor over link, an image appears somewhere on screen. By tweaking you can control placements. You can see demo, along with online version of this article at:
I've purposely made image appear over a text area so that you don't think it is a simple rollover image effect. The image actually appears above text.
Below lies code that of effect that appears above.
First HTML part that defines general link and division that defines placement of image. Take note of CSS definitions required to set z-index and "hide" attributes. Before testing code, remember to remove preceding dots that I have appended so that your email software doesn't read code as some "process-able" content.