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I'd suggest opening in an image editing program and then doing a Save As. You'll have a struggle here because of not introducing visible compression artifacts. If you have access to Adobe PhotoShop with ImageReady, then try Save for the Web, instead of SaveAs - the algos in there will help a lot.
In a worst case scenario, I'd open the graphic in Moz, do a screen capture, and create an entirely new image file from that captured data.