I've seen similar problems, often when different platforms are used for the creation of the graphic and the final display. I understand there are actually several types of JPEG data formats - most of which are not commonly used and Baseline or Progressive being the most common. But it's possible, depending on the software used to create/save the image that an odd data format crept in that Opera is choking on.
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.
some oddity that i thought i'd mention (oh yes.. i am looking back into the forums.. i found this place by... oh thats another story...) some browsers such as moz (i'm not so sure about opera.. i've only been using it again for a while (woo 7) and i havn't tested it) don't like jpeg files that were saved as CMYK format, A) they are larger B) they seem to be encoded different.. i noticed this after playing around iwht an art piece then saving it and wondering why i couldn't view it in my browser after uploading... looked at all the settings then noticed cmyk and changed it.. it worked.. so there is a tidbit to know...