|directory exporter   | 
directory := FileSystem workingDirectory / 'pictures' . 
directory ensureCreateDirectory. 
exporter := DOScreenshotExporter new. 
exporter directory: directory. 
exporter writeWorldAs: 'world.png'. 

with

DOScreenshotExporter>>writeWorldAs: aFileName
(directory / aFileName) ensureDelete.
PNGReadWriter putForm: World imageForm  onFileNamed: (directory / aFileName).

though this form might not work

DOScreenshotExporter>>writeMorph: aMorph rectangle: aRectangle as: aFileName
(directory / aFileName) ensureDelete.
PNGReadWriter putForm: (aMorph imageFormForRectangle: aRectangle) onFileNamed: (directory / aFileName).