Here's a video showing you just how to do that. Linking an image to a ConvertKit pop-up modal form. I've yet to experiment with how it is to have multiple images on the same page linked to different pop-ups. Until I play around with that, here's the video that shows you how to link one image to a form.
Also, if you want multiple forms to pop-up from buttons, here's the blog post with video tutorial on that:
024: How to Create Multiple ConvertKit Forms on My Squarespace Page?
Okay, now the video tutorial: (Copy code on the bottom of the video)
Here's the bit of code you'll need:
<a href="#ck_modal" rel="ck_modal"><img src="grab the image url from the browser by right clicking on the image"/></a>
Do share with me a link of where and how you incorporated this. I'd love to know.