Hi Claudio,
There is an in-built formatting specifier which BORIS recognizes for any output file, e.g. ns.saveimagefile("test_%sstep%_%Ha%.png"). Here %sstep% and %Ha% will be replaced by stage-step and applied field values when the file is saved.
see pg. 273 for a full example, this works as %dataname%, where dataname is any of the recognized output data handles listed in the chapter starting on pg 263.
Regards,
Serban