When adding screenshots they get downsized to max 630px, sometimes rendering the text unreadable. Right clicking and opening the image in a new tab reveals the image in original size. Is it possible to link the 630px 'thumbnails' to the full size images?
Absolutely, but you need to add the link manually as the toolbar doesn't have that functionality.
Taken from What's the recommended syntax for an image with a link?, the syntax looks like this:
[![alt text]] : http://meta.stackoverflow.com/users/44330/jason-s : http://www.gravatar.com/avatar/dd57e..dfd07?s=128&d=identicon&r=PG (hover text)