1) Making images clickable is basically to wrap them into <a></a> tags. I believe, you already know that, so I am not sure what piece of advice I can give you here aside from just adding <a></a> tags.
2) In order to add images to Order Items section in General info tab, you need to decorate the \XLite\View\ItemsList\Model\OrderItem viewer class and add new image field to the defineColumns() method similar to how it is done here:
http://kb.x-cart.com/display/XDD/ItemsList+in+admin+area
Please, let me know if it makes sense to you.
Tony.