Recently, Firefox has started blocking incoming photographs within emails, even those from trusted friends.
If I open the email in Internet Explorer the images are there within the email.
Which web based mail service are you using?
If images are missing then check that you aren't blocking images from some domains.
If an image in the list is grayed and there is a check-mark in the box "Block Images from..." then remove that mark to unblock the images from that domain.
Make sure that you do not block third-party images, the permissions.default.image pref should be 1.
There are also extensions like Adblock Plus (Tools > Add-ons > Extensions) and security software (firewall, anti-virus) that can block images and other content.