This should help.
Improve PowerPoint's GIF, BMP, PNG, JPG export resolution
http://www.pptfaq.com/FAQ00052.htm

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Michael Koerner
MS MVP - PowerPoint
i used this exported jpg(slide) from ppt in my Vb application to show it on
full screen (1280*800). early to installation of service pack 1 of 2007 , app
working fine for different resolutions.
after the installation of service pack 1 , image is exported from powerpoint
at higher resolution does not work
this is vba code of export menthod :
slides.export filename , fileformat , width,height
if height is or 800 and width 1200 then my exported image is not full
image with respected to the resolution set .. instead it defaults to 960*720
and remaing wth black color.
i tried rnr pptools ,still my exported jpg shows only 960*720 ,if i change
to higher resolution,still exporting @ 960*720
> This should help.
> Improve PowerPoint's GIF, BMP, PNG, JPG export resolution
[quoted text clipped - 13 lines]
> Powerpoint 2007 with service pack 1 installed . how to get good jpegs at
> higher resolution with out shrinking the image...
Steve Rindsberg - 05 Feb 2008 04:34 GMT
> i tried rnr pptools ,still my exported jpg shows only 960*720 ,if i change
> to higher resolution,still exporting @ 960*720
Yes, I'm afraid so ... because of the same bug you ran into with your code, Image Export limits
the maximum resolution from PPT 2007/SP1 to the resolution it will support w/o mangling the
images.