List of images with checkbox (user registration)
You can do that by the following. 

Go to Configuration » People » Account settings

Add a Field type : List (text) and WIDGET: Check boxes/radio buttons

In Allowed values list  textarea add key|&lt;img src ='/your/path/to/image' /&gt; 

Enter one &lt;img&gt; per line and set Number of values as Unlimited (to appear as checkboxes)

That's it. I tested it and working fine.

Add key whatever you want.You may have to make minor CSS changes in order to align checkbox and image.

Hope this helps.