Last updated on July 15th, 2015 at 03:44 am
I receive this error message while using wkhtmltoimage.
In Mac, it it not allow you to use Preview to save to other format.
It will display cannot write, or enable to save.
The fastest way to solve this problem, can be done in Automator, you can use Photo -> Flip Image -> Flip Image then save or convert to other format.
Then your PNG error will be gone.