In Outlook, View > Auto Preview?

It is not wise to use Auto Preview. Both Outlook and Outlook Express can auto preview an email and image attachments when the email is highlighted, but because of the risk of viruses within images nowadays, it is recommended that all attachments are saved to a temporary folder, (the desktop would do), and scanned with your uptodate anti-virus checker before opening them.

If you have Auto-Preview selected and you still cannot see images, then it might be a setting in Internet Explorer, > Tools > Internet Options > Advance tab, then scroll down to Multimedia and select the "Show pictures" option.