sometimes you would have to see, you may be just standing beside the official photog or a person from the press. this would cause similarity between your photos, you would need to have enough points to prove that the pic is yours before goin to sue them like tat is mention.
if you really find that both are same, go to the company first then if things get worst go to court and charge them under the copyright infringment law. you may get high compensation if you win*not sure also*, cos sg is promoting good copyright practise.
It all depends on which site you uploaed those picts to.
If you uploaed to Canon's site (eg only), they may have already gotten you to sign an agreement to say they now legally own the rights to whatever picts you uploaded. They can now legally resell those images to print publications for them to use.
Not sure about online album sites (eg Flickr). Would advice you to go back & read the agreements you agreed to without reading when signing up for that site's account.