Hi Jim, the image number is displayed at the bottom of the entry page under the "Record Information - Details" section where it says "Image Number: ". A link can be created to the image directly using the following url:
https://central.bac-lac.gc.ca/.item/?id=IMAGEID
(just replace the "IMAGEID" with the actual image ID)
You can download a JPG of the census image directly from the entry page. Click the "download" button right underneath the image, tick the box, and click "download file". Or, if you view the image using the URL I mentioned above, you can right click the image and "save as..." and it will save it as a JPG. (If you right click the image in the entry screen it saves it as a PNG instead).
Helpful tip: Ancestry uses the same image ID for most of their Canada census images as Library and Archives does. So if you find for example a 1931 census entry on Ancestry you can generate a free link to the image at Library and Archives Canada using Ancestry's image number.