as it seems difficult to find good examples of sites using longdesc image descriptions, this page serves as a demonstration for the firefox longdesc extension (see the related experiment).
after installation, right-click on the following images for the browser's context menu to view this extension in action.
the image depicts a default icon of Mozilla Firefox; it's a box of crayons, meant to signify images and graphics; there are three crayons — one for each primary colour: red, green and blue — and the red crayon is lying in front of the box. the image was based on a small graphic which was subsequently enlarged 200%, making individual pixels visible and giving an overall blocky, pixelated impression.