Assuming you have legally obtained the C3725-adventerprisek9-mz.124-15.t5.bin file, here is how to set it up in GNS3 (Windows/Mac/Linux).
With the firmware image validated, Alex proceeded to flash the Cisco router. The process was a bit rocky, with a few unexpected reboots, but ultimately, the new firmware took hold. The router roared back to life, its lights flashing with renewed vitality. C3725-adventerprisek9-mz.124-15.t5.bin Download