Desktop Picture is Cropped
I have a really nice picture of the grandchildren in iPhoto that I want to use as my desktop picture. When I do this by - File - Share - Set Desktop Picture - instead of getting the photo to fill out the screen of my 27" iMac perfectly, I find that the picture is cropped on the top, bottom, left side but strangely enough, not on the right side. What gives? I thought that the Set Desktop Picture would give me a perfect reproduction on the iMac's screen but this is evidently not the case. Any steps I could take to get the desktop picture that I want? I have the original 5K 27" iMac running 11.7.3 so quite an ancient setup but I don't think that this should have anything to do with my problem.
iMac 27″, macOS 11.7