Exchanging ppt files with PC - graphics issues

Question

I often exchange PowerPoint files with my PC-using co-workers. In the files they send me, some graphics appear as solid black boxes. In the files I send to them, some graphics appear as solid white boxes.
They have tried copy/paste and insert graphics, but we still get inconsistent results using both methods. Sometimes it works, sometimes it doesn't.
How can we avoid this problem? Any ideas?
Powerbook G4   Mac OS X (10.4.3)  

Answer 1

To iFelix - Most of the graphics are jpg files. The latest twist is that my co-workers get a message that they need quicktime in order to view the graphics in my ppt files - even though they are not movies - just still jpgs.
To Stephen - It turns out that my company has some additional security layers that would have to be peeled away in order for me to make a vpn connection from my Mac and they're not willing to do that. At least, that's what their PC IT geniuses tell me My solution is to make the vpn connection from Virtual PC. My need for the vpn connection is very infrequent, so it's not a big deal. It would be a hassle and very slow if you needed to work on a vpn daily.
Powerbook G4   Mac OS X (10.4.3)