You need to install the libX11 package:
$ rpm -qf /usr/lib/libX11.so.6
$ yum -y install libX11
One more thing though: if you don't know how to find and install a library package, care to share why you are trying to compile a piece of software that is officially packaged for Fedora 13 in the most recent version?
$ yum info gpicview
Name : gpicview
Arch : x86_64
Version : 0.2.1
Release : 3.fc13
Size : 93 k
Repo : fedora
Summary : Simple and fast Image Viewer for X
URL : http://lxde.sourceforge.net/gpicview/
License : GPLv2+
Description : Gpicview is an simple and image viewer with a simple and intuitive interface.
: It's extremely lightweight and fast with low memory usage. This makes it
: very suitable as default image viewer of desktop system. Although it is
: developed as the primary image viewer of LXDE, the Lightweight X11 Desktop
: Environment, it only requires GTK+ and can be used in any desktop environment.