(Option 1) As Vic Watt said if you make the image yourself then you can do what you like with it. But that means you need to visit NARA or another facility that has a physical copy of the census microfilm and make a copy of the page you want.
There are many reasons that Ancestry and FamilySearch (Plus others) claim copyright for their census images.
First, the image you see on their database is from their privately owned copy of the census microfilms.
Second, the image on their database is not the image you see on the microfilm. The have taken their microfilm roll and cleaned it. Then as each image is made, they subject it to various tools to enhance and make clear. Each image is unique.
Third, they have arranged those enhanced images from their personal copy in a particular way that creates their unique database,
(Option 2) Now for your, our, purposes there is an aspect of copyright that is called Fair Use but it is a fine balance. You can take a portion of their census image and use that to highlight the information that you wish. For instance, your ancestors are on line 21-28 of page 19 of District/Town X of New York 1880 census, you can grab a screen clipping of just those 8 lines and it would be (in all likelihood) considered Fair Use. Mainly because you have only used a small portion to illustrate a particular family; and, you have not used their image in a way that interferes with the value of their image.
(Option 3) Locate the microfilm roll number for the census in question, and find it on the website Archives.org. Grab a screen clipping of the page or section that you want from their viewer. Or download the roll in one of the formats offered and make copies of the images from that download. The microfilm rolls on that site were digitized from a collection of rolls purchased by Allen County Public Library and is available free of copyrights. But the images are 'as is' from the roll.
As you obtain, 'free and clear' images of the census please keep in mind the US History sub-project for Census Images. We are a small group but our goal is to do this very thing of providing census images for WikiTree to use.