This issue can occur with Mercury 4.0.0 or later when accessed via remote desktop on Windows. Images produced via File->Save As can appear compressed horizontally compared to what is viewed on screen. Images saved in this way when used locally (i.e. not via remote desktop) should be unaffected.
In order to save an image when accessing via remote desktop we would recommend instead using the File->POV-Ray Image option instead, which should not suffer from the same issue.