Hrm, I don't know where you got this information, but it seems to be incorrect unless I am misunderstanding your meaning. I thought perhaps you meant if you physically set copyright information into your image the thumbnails and crops created wouldn't retain those, but I just tested it with an image and if I set the copyright (using Photoshop) and upload to Flickr, the copyright remains embedded, intact on the smaller sizes as well.

Not that I typically embed the copyright information my images before posting them online anywhere, though it occurs to me belatedly that it may be a good idea for some of them to make sure there's an easier way to track that I own the image. But I think technically the images would retain copyright anyway, simply as you created it and didn't specifically give those rights away? I don't study law, though, so I could be wrong.