If you are looking for a simple PNG viewer for Unix which works under
XWindows, without having to install megabytes of stuff, there is a
very simple XWindows based viewer in the source codes available in
the /contrib/gregbook/ directory of the source codes of
libpng called rpng2-x. You need to compile it yourself
but it is only one file. There is also a viewer
called rpng-x, but this is less adequate since it doesn't
refresh the screen.
You can download libpng from here.