You can use HTML in the radio button label, so you can insert your img tag there, before the text. Then you need some CSS to hide the radio input (if you like) and style the text, and the “selected” “unselected” states for the image.
Or, you can use this plugin which makes this very easy:
Tanks for your help, the plugin is interesting but I tried to open the URL on my mobile, and it’s quite slow. I have been having the same issue with thumbnail images on radio button with different plugins. Any tip to make sure the form displays quickly ?