Any errors encountered during operation, e.g. invalid parameters, missing images, failure to generate the PDF etc, will cause Image to PDF to exit with a failure return code value greater than zero (NB a return value of zero indicates success). Additionally, text detailing the error will be output to the log file if it has been specified using the -Log option.
If Image to PDF is being used in an automated way without any direct monitoring then it is recommended that the -Log option is used. This will ensure that any output (including both errors and information messages) will be appended to the log file of your choice giving a valuable post mortem of any errors.