Adding photo signature line

I am following instructions to email photo my myself. Have saved the photo as both a jpeg and a gif. After right clicking, I do go to signature box in setting and "insert photo". When I enter the URL, I get the following message.
"We can't find the image at that URL
Please check the address for typing errors.
Make sure the image is public. If your image is protected by a password, or on an internal network, we will not be able to read it.