I discovered today that PPTX SlideEx.GetThumbnail() renders images incorrectly if the slide contains an image and “Color Mode” applied. Instead of grayscale output (as demonstrated in the sample provided), the image will appear full-color. I suspect this might occur with other filters as well.
Thanks for your interest in Aspose.Slides.
I have investigated the issue in detail. I have observed that originally the image in the slide is colored and you have just altered the visual properties of image from PowerPoint. Aspose.Slides for .NET reads PixelFromat to be Format32bppArgb, which is colored image. However, an issue with ID 21464 has been created in our issue tracking system to further investigate and resolve the problem. This thread has also been linked with the issue, so that you may be automatically notified, once the issue is resolved.
We are sorry for your inconvenience,
The issues you have found earlier (filed as SLIDESNET-21464) have been fixed in this update.