I installed libpng in my /sw/lib directory, but I still get the same error message

Listing my lib{Xm*,png*}:
/sw/lib/libXm.3.0.0.dylib /sw/lib/libXm.dylib /sw/lib/libpng.dylib /sw/lib/libpng15.15.dylib /sw/lib/libpng16.la
/sw/lib/libXm.3.dylib /sw/lib/libXm.la /sw/lib/libpng.la /sw/lib/libpng16.16.dylib
/sw/lib/libXm.4.dylib /sw/lib/libpng.3.dylib /sw/lib/libpng12.0.dylib /sw/lib/libpng16.a
/sw/lib/libXm.a /sw/lib/libpng.a /sw/lib/libpng14.14.dylib /sw/lib/libpng16.dylib

Any other thoughts? Should I purge some of the multiple copies of libraries?

By the date, my guess is that these are the libraries that were added/updated when I did so last; any particularly suspicious characters?:

libgsl.la
libgslcblas.la
libgsl.dylib -> libgsl.0.dylib
libgslcblas.dylib -> libgslcblas.0.dylib
libgsl.a
libgsl.0.dylib
libgslcblas.0.dylib
libgslcblas.a
libgmp.dylib -> /sw/lib/gmp5/libgmp.10.dylib
libgmp.la -> /sw/lib/gmp5/libgmp.la
libgmpxx.dylib -> /sw/lib/gmp5/libgmpxx.4.dylib
libgmpxx.la -> /sw/lib/gmp5/libgmpxx.la
libfreeglut.la
libfreeglut.dylib -> libfreeglut.3.dylib
libglut.a -> libfreeglut.a
libglut.dylib -> libfreeglut.3.dylib
libglut.la -> libfreeglut.la
libexpat.la
libexpat.dylib -> libexpat.1.dylib
libexpat.1.dylib
libexpat.a
libcloog-isl.la
libisl.la
libcloog-isl.dylib -> libcloog-isl.1.dylib
libisl.dylib -> libisl.6.dylib

thanks,
pt