Use an image editing software like Paint, GIMP or Photoshop if you're using a PC.
GIMP is free and works on MAC, Windows and Linux, so I'll link that for you. Just remember there are other options if you end up disliking it.
https://www.gimp.org/downloads/
I don't know much about editing images on Android or IOS, but if that's what you're using, I found a free app called Snapseed which looks like it would work.
https://play.google.com/store/apps/detai...e.snapseed
https://itunes.apple.com/us/app/snapseed...38619?mt=8
Then you upload the image to a site like
https://imgur.com/ and add it to your signature through the user control panel.
Lastly, if you need any help, I'll link a couple of video tutorials.
LEARN GIMP IN 30 MINUTES
Snapseed - Top 5 Tips & Tricks