Using *office is not always an useful way to manipulate a PowerPoint file directly.
I assume that you do not want a programmatic solution. If you did, you might consider using Apache Poi, which provides Java libraries for manipulation of Microsoft Office files.
MANIPULATING AN IMPRESS BINARY
To use a *office, product, what you need to do is open the *.ppt (or .pptx) in and save it as an ODF Presentation (.odp).
Using a Zip utility, you can open A COPY OF THE *.odp and look for an images subdirectory. (There may be more than one.)
You can delete them wholesale or selectively delete the ones that are obviously the high-resolution ones.
Close the modified Zip, rename back to *.odp if necessary, and re-open it in *office Impress. You may receive warnings about the document being damaged. Let the software attempt to recover the damage. You should have something that doesn't have the images any longer.
USING SKYDRIVE ON-LINE POWERPOINT TO MODIFY THE DOCUMENT
If your problem is that *office Impress won't open the .ppt/.pptx in the first place, or the *.odp modification doesn't work, you will need to find a way to delete the images using Microsoft PowerPoint software. One way is to get a Skydrive account (requires a Windows Live ID), upload the PPT file there, and edit it on-line using your browser. This is also a general way to view a .ppt/.pptx file when you don't have Microsoft Office software. Not all features of PowerPoint are preserved in the SkyDrive editor; you should be able to make the changes you want.