Photos don't appear on some computers

Hi,
I have helped create some PPTs for my company. We show these PPTs on
different computers depending on the venue, using whatever laptop is
available. Sometimes the PPT is e-mailed, sometimes it is just shown from a
jump drive. The PPTs were created in 2000 and 2003, but we've had a problem
with one particular photo not showing up on some computers. Now it's
happening with another photo. We can't figure out the problem because we
don't always know what version of PPT or what version of Windows the computer
has. Is this a problem with tranferring the PPT from one computer to another?
All the photos and graphics are inserted the proper way (not copied and
pasted) and we only have had a problem with two of them.

By the way, I've read that sometimes a red X will appear instead of the
photo. This is not happening. What happens is that the photo frame is just
dark gray inside it.

Thanks.