Thank you for your interest in Aspose.Slides.
I have observed your comments and like to share with you that one can not specify the resolution or dimensions while generating an image. However, scaling factor can be defined like getThumbnail(1.0f, 1.0f) to get image of exactly the size of the slide and getThumbnail(0.5f, 0.5f) to get image of half of the slide size. Bigger scaling factor will of course improve the quality of image and that image may be resized as per requirements by using a third party application.
I hope this will be helpful. Please share if I may help you further in this regard.