Mac Mail Attachment Issue

Hi
I am sending emails using Mail (and Mac OS X 10.6.8) to clients using Lotus Notes and Outlook on PCs. I am attaching jpegs to my messages (using "attach" and/or dropping them into emails).
Unfortunately the clients cannot see the jpegs. This problem however does not seem to apply to word documents.
What could be causing this?
When I look in the Mail sent box, I can see the jpegs attached, and they do appear to be being sent properly.
Thanks!

 

PS: I also have set attachments to "Send Windows-friendly attachments" and "Insert attachments at end of mail"
Recipients report a white box but no image.
Thanks for any help!

PPS: Client (using Outlook on Vista) says that when mail received image could not be seen. However when replied to, it appeared...