There have been multiple discussions on this subject. Go to the Search G2G box above and type in 'copyright' (without the quotes) and you will know more than you ever wanted to! *grin*
Certainly, as far as the census images from Ancestry and FMP, you can't. Check their Ts & Cs.
You can link to them, but you can't put a picture up and give credit via a link. And I wouldn't advise linking to Ancestry - after all, your census record is going to be behind a paywall, so many people won't be able to access it!