Images not showing up

Hello

The header image in my email newsletter isn't showing up. It appears properly in the preview after it's uploaded but when I send a test email it disappears. I also sent a test email to my Outlook/Hotmail address and the image displays properly there. It's not a very large jpeg (it's 600 X 227px).
There's one more smaller jpeg in the email and that appears without any problems.

Do you have any idea what the issue could be?

Thanks

D Mustapha