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Unfortunately, this is not possible in your basic javascript-enabled browser GUI. You'll have to turn to HTML's favourite cousin, Flash.
With Flash, it is possible to create custom GUI elements, and with some effort it's possible to integrate them into your HTML forms. However I might warn you that it's not a good idea. If you're asking the user to choose an image, the best (and friendliest) ways are:
a) show a normal select box with the image file names or descriptions
b) show an array of clickable thumbnails.
c) show your thumbnails accompanied by a set of radio buttons.