Hi stevenh,
It is an interesting situation you describe.
IF the images were actually TOTALLY embedded, then you would NOT require the "original image files" which you say are "missing". Therefore, your embedded files were likely embedded as "linked" files - i.e. linked to disk-based filenames ...which are now "missing".
The second possibility is that the embedded images were not TOTALLY embedded NOR linked - i.e. they might have simply been embedded as PICTURES or DEVICE-INDEPENDENT BITMAPS. If this is the case, then there is NO way (that I am aware of) to export these embedded images (back) into their original file format. You NEED to have the ORIGINAL files. If they no longer exist, then I'm afraid you might be "out of luck".
OBSERVATION: It occurs to me that your intented course of action (to "link" the images insted of totally embedding them) will end up creating the exact same problem-situation which you now face - i.e. IF you link the images to disk-based files, you will potentially be faced with those disk-based files later being (accidentally) deleted - causing the "link" to be severed.
MY RECOMMENDATION: ...that you are FAR BETTER OFF to "TOTALLY EMBED" images within PowerPoint. There are TWO SIGNIFICANT advantages to doing this.
1) With the files being TOTALLY embedded, there are no disk-based independed files to worry about accidentally deleting. And appreciate that these (TOTALLY EMBEDDED) images are still "TOTALLY EDITABLE" in their own native format. This is done by "right-clicking" on the image, and then choosing "Open ______ Object", where "_____" is the software which was used to create the object. I use CorelDraw extensively, and therefore this would read "Open CorelDraw Object".
2) These TOTALLY EMBEDDED images take up FAR LESS SPACE on your disk. This is because PowerPoint has a special method of compression, or perhaps because PowerPoint is not the "originator" of the image, it does not have to save these embedded images in same way that the "originator" software does.
The "bottom line" on this, however, is that the TOTALLY EMBEDDED images (without any linking to disk-based files) are STILL TOTALLY EDITABLE by the software which was used to create these images, and INDEED these images ARE the VERY SAME IMAGES in EVERY respect as images which are saved independently in their "native file format". Furthermore, if you later want to save a copy of a (totally) embedded image to a disk-based file (in its native file format), you can do so at any time.
I hope this information is informative and helpful.
I would be interested in knowing what your decision will be regarding the option to TOTALLY EMBED images, or to LINK to disk-based filenames.
Regards, ...Dale Watson dwatson@bsi.gov.mb.ca