Some sites have a limit on the size of image that they allow you to upload, and will resize anything that exceeds that limit. Sometimes that resizing doesn't do much for the quality of the resulting image.
For sites like Flickr that allow you to upload full size images, and will display reduced sizes, always remember that the full sized image is available to anyone who might want to pinch your image for good quality prints or other nefarious purposes. That's one of the reasons I limit my uploaded images to a maximum of 750 or 800 pixels on the longest side.