Do you have Exchange server, Exchange client, or firewall/proxy blocking of image types or sources?
If so, you should amekt sure that none of those files are intercepted, or they will not go through or become unopenable.
Hope this helps!
Since they CAN be opened by MAPI client, it does not sound like anything is being blocked at the firewall level. However that does not mean that there is not a transport rule or something at the exchange server level regarding POP3 and imap accounts. I would bring this to the attention of your exchange admin who can check things from the server side and also determine if this is true for everyone or just for you for some reason.