Hi, I am also having problems with the ReCapthcha plug-in (v2). It gives an error message: "Your input did not match the captcha"! After investigating, it seems that the response value is not correct.
I am using ZenPhoto version 1.5.9! So installing 1.5.8 is not an option for me. What is the modification you made? If so, I may be able to implement it manually.