This happens when you have Outlook set to automatically download pictures from the Internet and when the web server hosting the image is slow or not available.
With newsletters from upcoming web services, this is more likely to be the case but it can also happen when an email contains an excessive amount of pictures or when you have a slow connection yourself.
Block, prompt and do not permit exception
To set your Automatic Download settings a bit more stricter go to:
Outlook 2003 Tools-> Options-> tab Security-> Change Automatic Download Settings…
Outlook 2007 Tools-> Trust Center…-> Automatic Download
Outlook 2010 and Outlook 2013 File-> Options-> Trust Center-> Trust Center Settings…-> Automatic Download
The safest configuration is as follows:
Don’t download pictures automatically in HTML e-mail messages or RSS items
Permit downloads in e-mail messages from senders and to recipients defined in the Safe Sender and Safe Recipients Lists used by the Junk E-mail filter
Permit downloads from websites in this security zone: Trusted Zone
Permit downloads in RSS items
Permit downloads in SharePoint Discussion Boards
Warn me before downloading content when editing, forwarding, or replying to e-mail