Hard to know. Expect the images are LINKED - that is they are on a webserver somewhere in the world, but not linked correctly so they do not show up. Another possibility is that the images have been removed by your anti-virus and/or anti-spam software. Another possibility is that your e-mail settings are set so that it does not automatically show images (good option to have set, so avoid changing it).
Has the e-mail come from a trusted source - i.e. a friend of yours, not just a corporate e-mail address?
If so, try Right Clicking on the little square and seeing if you get SHOW PICTURE or some similar option.
If that does not work, ask your friend to resend images as attachments. Ask them to send them one image per e-mail, so that you can check that you can open the attachments correctly.
Are the pictures important? Sometimes images get put in e-mails which are not important, but are used for tracking who opens the e-mail. Spammers use this to know which e-mail addresses are valid, and thus once they know you have read their e-mail, they send you a whole lot more.
So how you resolve this issue will depend a huge amount on how much you trust the e-mail address used to send you the e-mail (and also the content of the e-mail itself, as the e-mail sending address can be forged). If it is a personal e-mail from a friend of yours, and has included text as well as the images, just ask them to resend the images as attachments.
Hope that helps.