Follow the steps:
Open the image you want to change
Go to IMAGEImage --> MODEMode --> BITMAPBitmap
NOTE: If you're using RGB COLOR first you need to put in gray scalegrayscale (IMAGEImage --> MODEMode --> GRAY SCALEGreyscale, Discard color infoDiscard color info)
Click OKOk in FLATTEN LAYERSthe Flatten Lines message box
After that, in METHOD USE: 50% THRESHOLDMethod Use: 50% Threshold
That's all