libX11.so.6 Not found

You need to install the libX11 package:

$ rpm -qf /usr/lib/libX11.so.6
libX11-1.3.1-3.fc13.i686

Just go

$ 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
Available Packages
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.

I bet a more appropriate response would be to install libX11-devel...