PhotoJAR: Base provides numerous enhancements to the WordPress gallery and provides a framework for additional PhotoJAR plugins.
Current PhotoJAR plugins are listed at the PhotoJAR webpage.
Create the directory specified in the error message, and ensure that WordPress has permission to write in it.
Please contact me via the PhotoJAR website.
Include the version of WordPress you are using, current PhotoJAR settings, and details to recreate the bug if possible.