To use images as options in a multiple choice field, go to your Form Editor and from the Basic Fields section, add a Multiple choice field to your form. Then, in the Choices section from the Edit field panel, paste the URLs of the images you want to use, one on every line.
Make sure to have the hosted link of the image SSL secured (https).
For the images to display on your form, the links must end in the image’s extension (e.g. img/png).