I have tried to use zgv with PNG on 4 RH6.0 systems and it errors out.
I have tried re-compiling from the src.rpm and it gives me a:

zgv.c: In function `makedirxv332':
zgv.c:1711: warning: suggest explicit braces to avoid ambiguous `else'
zgv.c: In function `update_xvpics':
zgv.c:1842: `errno' undeclared (first use in this function)
zgv.c:1842: (Each undeclared identifier is reported only once
zgv.c:1842: for each function it appears in.)
zgv.c:1842: `EEXIST' undeclared (first use in this function)
zgv.c:1842: `EACCES' undeclared (first use in this function)
zgv.c:1921: warning: suggest explicit braces to avoid ambiguous `else'
make: *** [zgv.o] Error 1

And I have patched the the code with the patches that came with the src
RPM. (zgv-3.0-8). I have downloaded the 3.3 tarball and modified it
and compiled it successfully. It does give lots of explict braces warnings
but still compiles. And of course works. I understand that 3.5 will be out
in a few weeks as well. I just thought you might like to know.

The binary rpm seems to default for no PNG support and if you are working
for a GIF free environment that can be a real pain.