/usr/local/bin/mogrify is correct
As I posted above your mogrify is not allowing png images ( has not been set to have access to png libraries ) thus you can not upload png images. That is why your watermark has failed.
You can try converting the png watermark to transparent gif and removing png as an acceptable image type in photopost config or see if your host will solve the processor issue.
You no doubt saw my uploads to test this issue and the resolution on your install