Go into editing your Signature, and you'll see a little button on the toolbar with all the other buttons. The button pictured below is the one you want to click.
When you click this button, it'll ask for a http://
Delete the http:// that's already there. Then, enter this into it http://i445.photobucket.com/albums/qq172/LordOfDarkness_2008/DK1000Signature.png
Once you've entered that in, click okay. And make sure to save your edited changes to your Signature before leaving. The image should then appear as your Signature. If not, and you have any problems, let me know ^_^