Which screenshot gesture were you trying to use (there's two of 'em, IIRC
)?
(Okay, reading your post properly helps): I assume you tried the diagonal swipe. That's disabled in ImageViewer because swipes are bound to panning there. You'll want to use the other gesture: a single short tap on two opposite corners of the screen (so, two fingers in sync required) (emphasis on *short* on fast devices, because you want those to register as a tap, so, fairly steady fingers ;p).