Views: how to add rel attribute to multivalue image thumbnail
You can install Image URL Formatter module. This will give you "Image URL" as a formatter option on Views. 

So what to do is:


Add the image field, and Exclude from display. Set the formatter as Image URL
Add the same image field again. This one should be outputted as a thumbnail with Image Styles (perhaps), and have the normal "Image" formatter. Rewrite -> Output this image as a link, put the field added in step 1 to the link path.