Hi Roel In a normal radio button field when you choose the image option it adds an extra div directly after the <input> with the image source and an SVG icon. E.g.

<div class="frm_image_option_container">
<div class="frm_selected_checkmark">
<svg class="frmsvg"><use xlink:href="#frm_checkmark_circle_icon"></use></svg>
</div>
<img src="/wp-content/uploads/2021/02/iconfinder_face_Angry_evil_hatred_emoji_5241538.png" alt="Option 1" data-pagespeed-url-hash="1122291104" onload="pagespeed.CriticalImages.checkImageForCriticality(this);">
</div>

You could probably add that manually to a Lookup field set of radio buttons with jQuery or plain JS using a foreach loop and a list of your image URL's or it might be better to do it with PHP and the frm_setup_new_fields_vars hook or maybe the frm_filtered_lookup_options hook. Hope that helps.