I know it's been a long time since you posted this, but I thought I'd add a link to
this post by Paul Huson on the DFW thread where he mentions where the image came from and why he chose it.
I'm with you; I don't think he looks powerful. I still like the image, though. First, because I like the history behind it, but also because I tend to read the Devil card in terms of destructive impulses within ourselves, and this Devil actually elicits compassion from me. He looks misshapen and like he has no idea what to make of himself. The Frankenstein monster comes to mind.
I'm sure the original purpose of the Devil in a morality play (mentioned in the link above) was to hate an outside force, but since I read the card as an internal force, I could stand to have a little compassion for my destructive impulses, or someone else's, because that way of coping seems to do a better job of helping me not act on those impulses than being self critical does.