To complicate matters a bit, there is significant geometric distortion in the Voyager images so the pixel scale really isn't completely uniform across the image. Once the images have been geometrically corrected they look like this:Click to view attachment
This is a Voyager 1 image of Rhea from the calibrated and geometrically corrected Voyager volumes at the PDS Rings Node.
I don't think this is usually taken into account when the resolution is specified and the effect is small. Where the image above is widest (top/bottom), the FOV ~0.438 degrees if you omit the black 'frame' around the image and 0.424 degrees near the center because the image edge is 'curved'.