It (reversed) was also the logo for the Icelandic Steamship Co., on whose ship, SS Godafoss, I crossed the Atlantic twice in WWII. We encountered U-Boat packs, but were not among the sunken (did the Nazis see the flag?). Actually, the Godafoss (a sister ship to the one depicted in the poster) was eventually torpedoed and sunk (in 1944). Funny thing, I remember when the British occupied Iceland, in 1941, they stormed the Eimskip building, which was flying that logo flag. Guess it fooled them.
And here's an actual photo of the ship I sailed on...