Choose File | Options | Trust Center | Trust Center Settings and de-select "Don't Download pictures automatically in HTML e-mail messages or RSS items."
NOTE: this applies to all messages that you receive. If you would like to only download pictures from certain senders, you can add the sender to your Safe Senders list.
- Right click on the message.
- Select “Junk E-mail”.
- Select “Add Sender to Safe Senders List”. The pictures will show in emails from that sender.