[NTG-context] How to override the print resolution

When using \externalfigure it uses the print resolution for putting the graphic into the document. Until now all my graphics had a resolution of 300x300, but I just got one with a much lower one, which blew up in my document. I just set the print resolution to 300x300 and everything is fine again, but is there a way to override the printer resolution? -- Cecil Westerhof