Unfortunately our resident (and veteran... So old) Imperial Guard player Spicerack has confirmed this image as "FAKE".
Unfortunately it's apparently very old artwork from some old Epic 40k books, as Imperial Knights were playable in that system. Someone just have just put the white dwarf text over the image - nice try internet!
We're hoping some of these images surface soon - we'll be here to report them!
Alles gute,
TommyH