I tried to install gnome-screenshot on top of gnome-light, only to find out that there are no versions available that depend on pre gnome-3 libraries. Since gnome-3 is itself ~arch'ed, I think it would be a good idea to readd a pre-gnome-2 version of gnome-screenshot so that I'm not forced to make a big upgrade.
There has never been a gnome-2 version: http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/media-gfx/gnome-screenshot/?hideattic=0
*gnome-screenshot-3.4.1 (05 May 2012) 05 May 2012; Alexandre Rostovtsev <tetromino@gentoo.org> +gnome-screenshot-3.4.1.ebuild, +metadata.xml: New separate gnome screenshot package, formerly part of gnome-extra/gnome-utils. So you should be installing =gnome-extra/gnome-utils-2* with that. =gnome-base/gnome-2* has that in RDEPEND, but then again, gnome-base/gnome-light is supposed to be lightweight...