Hi Lucas,
One way to achieve this is to use JavaScript to insert the HTML for images into the label for each radio button option.
In the below code, the radio button is identified as q18 on the form, and has 3 options (options 0, 1, and 2 in order). Each radio button option has a label, to which an image is being appended. You should have the images hosted where any browser can get to it. The width and height tags are optional. You can choose to set the width and height in these tags or you can use CSS to style the images using the class radiobuttonimage applied below.
$(document).ready(function() {
$('label[for="Field18-0"]').append('<img src="http://imageURLhere" width="100px" height="100px" class="radiobuttonimage">');
$('label[for="Field18-1"]').append('<img src="http://imageURLhere" width="100px" height="100px" class="radiobuttonimage">');
$('label[for="Field18-2"]').append('<img src="http://imageURLhere" width="100px" height="100px" class="radiobuttonimage">');
});
I hope this helps!