Source: http://support.microsoft.com/default.aspx?scid=kb;en-us;827745

Source 2: http://www.ajronline.org/cgi/content/full/185/1/273#REF1

This step-by-step article describes how to change the resolution dots-per-inch (the dpi settings) of exported slides by adding a new registry value for Microsoft Office PowerPoint 2003 or for Microsoft Office PowerPoint 2007. Additionally, this article describes how to export a slide as a picture.

Note The following table shows the default settings for a slide that you export as a picture.

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Pixels Dots-per-inch (dpi)
960 x 720 96 dpi (horizontal and vertical)

Add the registry settings

To export a slide as a picture with a resolution greater than the default 96 dpi, you must add a registry value. To do this, follow these steps.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows
  1. Exit your Microsoft Windows programs.
  2. Click Start, and then click Run.
  3. In the Open box, type regedit, and then click OK.
  4. Expand the registry to the key for the version of PowerPoint that you are using:

    PowerPoint 2003
    HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\PowerPoint\Options
    PowerPoint 2007

    Note The registry key for PowerPoint 2007 will not work with Office 2007 Service Pack 1.
    HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\PowerPoint\Options
  5. With the Options key selected, point to New on the Edit menu, and then click DWORD Value.
  6. Type ExportBitmapResolution, and then press ENTER.
  7. With ExportBitmapResolution selected, click Modify on the Edit menu.
  8. In the Value data box, type the resolution value that you want, based on the following table.

    Note The maximum resolution setting that PowerPoint 2003 exports is 307 dpi.
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    Decimal value Pixels (horizontal x vertical) Dots-per-inch (horizontal and vertical)
    50 500 x 375 50 dpi
    96 (default) 960 x 720 96 dpi
    100 1000 x 750 100 dpi
    150 1500 x 1125 150 dpi
    200 2000 x 1500 200 dpi
    250 2500 x 1875 250 dpi
    300 3000 x 2250 300 dpi
  9. Click Decimal, and then click OK.
  10. On the File menu, click Exit to exit Registry Editor.

Export a PowerPoint slide as a picture

To export a PowerPoint slide as a picture, follow these steps:
  1. Start PowerPoint, and then open your slide presentation.
  2. Open the slide that you want to export.
  3. On the File menu, click Save As.
  4. In the Save as type box, select the type of picture that you want to export your slide as, and then click Save.

    For example, in the Save as type box, select one of the following:
    GIF Graphics Interchange Format (.gif)
    JPEG File Interchange Format (*.jpg)
    PNG Portable Network Graphics Format (*.png)
    TIFF Tag Image File Format (*.tif)
    Device Independent Bitmap (*.bmp)
    Windows Metafile (*.wmf)
    Enhanced Windows Metafile (*.emf)
    Note You may also want to change the location where you save your picture by changing the location in the Save in box. You may want to change the name of your picture by changing the name in the File name box.

Limitations

When you set the ExportBitmapResolution registry value in the registry to change the export resolution of a picture in Microsoft Office PowerPoint 2003, there is a limitation that you must be aware of. The maximum resolution setting that Office PowerPoint 2003 exports is 3072 pixels measured from the longest edge of the slide. For Example, the standard 10x7.5" slide would have a maximum effective DPI value of 307. (10x307) = 3070. The measurement of 3070 pixels falls within the 3072 limit. Any DPI settings higher than 307 for the standard slide would revert to the limit of 3072.
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Decimal value Pixels (horizontal x vertical) Dots-per-inch (horizontal and vertical)
307 3072 x 2304 307 dpi
Note The limitation of 3072 pixels applies to programmatically saving the slide as well. Code that is designed to export the slide at a given pixel measurement will also be limited to 3072 pixels.

For example, ActiveWindow.Selection.SlideRange(1).export "c:\filename.jpg","JPG",4000,3000 will result in a graphic being programmatically exported at 3072x2304 pixels.