In your Makefile.am, add
dist_pkgdatadir_DATA = foo.png bar.png
Your PNGs will be installed in $(datadir)/$(PACKAGE)
You can customize the installation directory
pngdir = $(datadir)/$(PACKAGE)/img
dist_pngdir_DATA = foo.png bar.png
Please read automake manual:
http://www.gnu.org/software/automake/manual/html_node/Data.html#Data
http://www.gnu.org/software/automake/manual/html_node/Install.html#Install
http://www.gnu.org/software/automake/manual/html_node/Uniform.html#Uniform
http://www.gnu.org/software/automake/manual/standards/Directory-Variables.html#Directory-Variables
Regards
--
Yann Droneaud