Since you've uploaded that gif here it's easier to link it:
- Right click on your attachment and pick "Copy link address"
- On your signature screen, you can use Insert Image (Ctrl+P)
- Paste the link of that attachment inside the URL bar
- Resize the image to however you like it
- You're done.
Also you've linked a gif in your current signature too so it should be pretty easy to do the same thing you've done before.