You can change your settings for
who can download your photos here. If you change it from anything other than "Anybody" Flickr puts a clear .gif file over your images to
*discourage* downloading. Still, it will make it so that people can't grab the full size images that you upload. There's no 100% protection on the internet, but that's a choice / tradeoff you have to make if you want to show your photos online.