Thanks for your interest Aspose.Slides.
I regret to inform you that Aspose.Slides for .NET does not support presentations from Google Apps Engine. Aspose.Slides for .NET supports reading presentation files as memory stream from web servers and then accessing that memory stream in Presentation class object. I have tried to access the presentation generated in Google Apps Engine and unfortunately unable to access that.
Thanks and Regards,
Actually, Aspose.Slides for Java allows you to convert PPT/PPTX to JPG thumbnails. The support for webservice using GAE is unavailable since the format of GAE based presentations is not recognized by Aspose.Slides for Java as well. In my suggestion, you may develop your own custom application for presentation generation on some webserver as GAE and then using memory streams access those presentations and perform your desired operation afterwards.
We are sorry for your inconvenience,