Just enter some text click the 'Go' button. A danger sign will be generated for you.
See also: Safety Sign
(Note: do not link directly to the image generated on this site - it won't work. You are free to download the image, copy it, put it on your own website, or distribute it however you like. If you don't have a website to store it on, there are plenty of free image hosting sites available.)
Photo credit: topulatis