The one that started it all! This is H.R.Giger's painting "NECRONOMICON IV". As legend goes (heh) Ridley Scott was given one of Giger's Necronomicon books in the early-early days of ALIEN pre-production. Scott begins flipping through it and comes to the page with NecroIV and Bam!!! He instantly says, "THAT is the monster for the film! Get this dude on the phone!" (Or something along those lines). The rest is history.
I had this image as a poster when I was a teenager. Needless to say, my mom was thrilled. Hmmm, maybe it's time to ship that badboy out to LA and have it grace our living room!