It seems as if you're using the link to your hard drive, upload to http://imgur.com/, and click the insert image option, then insert the direct link.
let me see: C:\Users\Gabriel\Desktop\gabeli's Signature.png[\img] [img]C:\Users\Gabriel\Desktop\gabeli's Signature.png
right click the image you want then select "copy image URL" go to your signature and click on the tree icon. Paste the URL into here