libpng is from the "l" (lowercase L) disk set in Slack. But I do not know about the version you posted, with number prior to the so designation. Slack 8.1 provides libpng-1.2.1. The following is what my system currently has:
bash-2.05a# ls -l /usr/lib/libpng*
-rw-r--r-- 1 root root 197288 Feb 14 2002 /usr/lib/libpng.a
lrwxrwxrwx 1 root root 11 Oct 12 18:41 /usr/lib/libpng.so -> libpng.so.3
lrwxrwxrwx 1 root root 18 Oct 12 18:40 /usr/lib/libpng.so.2 -> libpng.so.126.96.36.199
-rwxr-xr-x 1 root root 183398 Feb 14 2002 /usr/lib/libpng.so.188.8.131.52
lrwxrwxrwx 1 root root 17 Oct 12 18:40 /usr/lib/libpng.so.3 -> libpng.so.184.108.40.206
-rwxr-xr-x 1 root root 188678 Feb 14 2002 /usr/lib/libpng.so.220.127.116.11
After the package is installed, or if it is installed, you can locate more info in the /usr/doc/libpng-1.2.1 directory tree.
Sometimes packages are built to use libs that the author had installed but may not be a part of the standard distribution of the library. Or they had some beta package installed at some time in the past it the library changed over time. I have had some limited success in just trying to create a link to the version that I have. Like you might try the following and see if it works OK or not.
ln -s /usr/lib/libpng.so /usr/lib/libpng12.so.0
That would create a symbolic link for the file that is being searched for but redirects it to the file that you have. If it doesn't work, you can always delete the sym link.