I use Mozilla Thunderbird for a mail client. For security purposes, they defaulted the images to never be displayed and offer an option to display pictures only for select entries in your address book. Not wanting to clog my address book with hundreds of entries for email that I don't actually use, I prefer to accept the security risks and default images to be displayed. Here's how to do it.
Open the Config editor (select "Options -> Advanced -> Config Editor) and toggle the preference, mailnews.message_display.disable_remote_image to "false". Note that this is not recommended since it will allow loading of all remote images and spammers will be able to verify your e-mail address.