based on the choice documentation it is posible to use images as options instead of text.
I'm using currently Moodle 3.1 and it looks like that this option is not available anymore. At least in my Moodle instance after trying with different file formats, starting from jpg, gif, png and using this code:
<img src="https://<URL>/file.jpg" alt="file.jpg>
Does anyone have any idea why it does not work?