While this is not an answer but a work around that works for me.

You need to have both a small image , and a larger version of the image imported into your report.  Then use the following code to display the large image when exporting to PowerPoint.

The following code should be used in the rectangle's `BackgroundImage.Value` property

    =iif(Globals!RenderFormat.Name="PPTX","LargeImage","SmallPage")