Displaying assets from Craft's cart AJAX response
The cart response definitely does not include direct access to fields on the product.  You will need to make an extra request for the information...of course you'd need to make an extra request for the image itself anyway.

One easy way of implement what you want is to simply update your cart view by ajax instead loading a template into it (rather than targeting values and using the response.cart object).  In that template you would just get the cart as normal, and therefore have access to all the product fields via that as you loop through the lineitems.