I thought it was more simple at first now its like uh… Kinda hard lol.
Not a computer forum wiz-kid so I have no idea how to do it, thanks for your help if you intend to do so. 
I’d assume you’d insert an image from imgur or a site like that the same way you do in a post. Making sure you have the .jpg/.png/whatever at the end, and surround the link with and
The first thing to do is to find an image host like Flickr or Imgur or Photobucket. Then you upload your image to one of these and get the address of the photo. It must be the photo itself not the page the photo is posted on. Right clicking on the photo and selecting Open in new Tab should get you the literal address of the image - as Connor said above you’ll know you’re on the right track if the address ends with file type of the image.
Then you simply surround the address of the photo with img tags like this: address to your image and put it in your sig.