Right-click on the Video Preview and pick "Display at project size" . Also set preview quality to best.
Set your project property "Interlace" format to "progressive" and the (advanced property) "Deinterlace method" to "Interpolate" so you won't get any interlace artifacts in your images. Make sure to set the interlace property back before you render your project, you only want progressive for the still capture step.
Use "Save Timeline snapshot to file" because it deals with pixel aspect ratio, while "Copy Snapshot" does not.