Zazen shite hito ka hotoke ni narunaraba.
Please wait for our experts, but I'd translate this roughly to: "Through zazen, you could become either a person or a Buddha." Meaning you can't reach enlightenment by only focusing on the outer form of zazen and neglecting the spiritual aspects of seeking the right path.
Thank you!Embedded in this image and the accompanying phrase, is the idea that if zazen was all that was required, then even a frog could become human (because the frog is basically sitting in a zazen-like position all day). So it is a mild admonishment against just following the basic postures of zen.
In other words, what Thomas said above.