The thumbnail image of a slide does not come up well for slides whose background contains Gradient of preset type.
Along with this , am attaching the PPT which contains a slide with backgound of gradient preset type and the image generated for that slide and the code used for it. You can notice the difference between the actual slide and the generated image.
Please look in to the problem and provide us a solution.
Thanks for reporting, we will fix this bug as early as possible and let you know.