Click File> Save Image > then instead of clicking Computer click on Imm.io > Click Ok > Then instead of clicking Ok again select view. Then copy the image and put it in your Sig. Hope that helps.
The website you resized your image and got the image url from doesn't work on here I guess. (Picresize.com) Just right click the image, Save image as.. and then put it in your pictures on your computer. Then upload it on imgur then use the Image url from THAT, and not from Picresize.com. That's if you're using Chrome.^