Two security issues exist in version 0.7.6,
According to http://optipng.sourceforge.net/ :

"
You are strongly encouraged to upgrade to the latest version 0.7.7.
Here is a list security-sensitive issues that affect the previous versions: 

Joonun Jang reported a buffer overflow vulnerability in the GIF decoder, discovered by a fuzzer developed by the SoftSec group at KAIST. All versions prior to 0.7.7 that support GIF files (i.e. from version 0.5 to version 0.7.6) are affected.

Jaeseung Choi reported an integer overflow vulnerability in the TIFF decoder. All versions prior to 0.7.7 that support TIFF files (i.e. from version 0.5.3 to version 0.7.6) are affected.
 "