Census records are kind of a bad example to use for this discussion as they are available from so many places. But as I said, I do tend to save image files and transcripts to my own computer, along with the info on thew web site where I found them. Its always better if you have a source for them that is more or less permanent: Ancestry or Family Search for example.
I wouldn't be inclined to crop any of the images if they are documents. You never know when something you cropped away might be important. Of course if you want to crop a copy of the original image thats ok, but I would keep the original version uncut, just in case.
I totally agree that you don't need to go out and snap your own photos of cemeteries, etc when you can find them online. The polite thing to do is ask permission. Some people get snippy if you don't, but I'm not one of them. I feel pretty stoked when I see people using my photos of our mutual distant ancestors. But when you ask permission, give them a little background on who you are and how you are related as they might have more information for you.