Right click on the image that you want to copy in my signature or someone else's. Click properties. Then highlight the URL address and right click copy. Now go into your User CP and click Edit Signature. In the box that comes up there is a picture in the toolbar that looks like a postcard with a mountain range on it. Click on that and clear the address line. Then paste the URL address that you copied previously.
Post back if it doesn't work for you. People with different browsers seem to have different issues and someone will probably post with a different method.