Operating system: Windows 10 64 bit
Graphics card: Nvidia GTX 750 Ti
Short description of error
Reference Images have the wrong aspect ratio, if the Limit Size in the user preferences is set lower than the longest side of the image.
Exact steps for others to reproduce the error
Set the Limit Size in the user preferences, under Viewport > Textures to a value greater or less than 1024.
With values greater or equal to 1024, the reference image is rectangular.
With values less than 1024 the image becomes square.
Optionally, file working in Blender 2.79