eliminate small polygons of a PNG with gdal_sieve

I want to eliminate polygons wich consist of less than 3550 px in this PNG . I tried using gdal_sieve but there seems to be an error.

gdal_sieve -st 3550 E8.png -of PNG E8sieve3550_4.png
ERROR 6: GDALDriver::Create() ... no create method implemented for this format.

Traceback (most recent call last):
  File "C:\PROGRA~1\QGISBR~1\bin\gdal_sieve.py", line 165, in <module>
    dst_ds.SetGeoTransform( src_ds.GetGeoTransform() )
AttributeError: 'NoneType' object has no attribute 'SetGeoTransform'

I also tried "PNG" instead of PNG and changed the order of the parameters..