I'm using GM: Studio and what I get after using the extension's command (I've inspected also the example project and it seems I'm doing nothing wrong) and after saving the image is a completely black image (interpreted as completely transparent image in Windows's images viewer) with the resolution of the canvas in witch i'm running the HTML5 game exported.
The strange thing is that your demo works just fine on the same browser (Chrome) and with the same simple call you show. Am I maybe forgetting some simple basic concept or mechanism in GM that could interfere with my project?