Using an image as a checkbox or radio button?

Sure you can use an image as the label for a radio button, to do this you will need to check the 'enable values' option on the properties tab of the radio button field. You can then add your html code that will display the image to the label not forgetting to write a meaningful name in the value box so you know what image they choose when you get the email notification.

If you don't want the actual radio button to display as well as the image you can add some css to do that but you may want to add a border or something to identify which image they have selected. You will need to add the class 'hideradio' to the css class name on the radio buttons advanced tab

.hideradio input[type="radio"]{display:none;}
body .gform_wrapper ul.gfield_radio li input[type="radio"]:checked+label img{border:solid #000 1px;}

Could you post a link to the page with your form on it, that second style should add a border to the image when it is selected. Even though you can't currently see a visible sign that one of the images has been selected can you check the forms entry in the admin section to see if it has recorded the selection anyway.