Solution
I built a Flex component called PopUpThumbnail that is very similar to HTML/JavaScript lightbox components.
Here's an example application that I created with it to displays Flickr images.
OTHER TIPS
Add a mask over the top of the whole application with the content area in the middle and the semi alpha area around it. Clicking on the surrounding mask removes the layer, and you can interact with the content area as normal (make it look like a popup rather than actually using one...)
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