Chances are it's your webmail settings!
Some webmail sites, including GMail and Yahoo Mail block images, especially images that have been hotlinked from other sites. They say they do this to protect you from malware, but I suspect it's more likely that they are trying to block 3rd party advertising!
Anyway, here's what to do in GMail and Yahoo Mail:

GMail:
Below is a message from member Demon which includes hyper-linked images:


1: This is where an image or images should be, but it they are blocked by GMail.
2 and 3: Click 2 to show the images this one time. Click 3 to always show images from this person.



Yahoo Mail:
Below is the same message, which I forwarded to my Yahoo account from GMail:


1: This is where the image or images should be, but they are blocked by Yahoo Mail.
2:Click here to see the images in this email only!
3: Click here to change your settings so the images will always appear.



If you clicked 3, you will now see something like this:



4: Click here to display the spam options on the right.
5: If you want to always see the images, chose this option
6: Be show to click the "Save Changes" button!

AN EXTRA NOTE ABOUT YAHOO MAIL:
Yahoo Mail is notorious for sending Google Group mail to the Spam folder. If you are using Yahoo Mail, both on the webmail site or downloading it, go to the webail spam folder about once a month and check that there are no Google Group mails in it. If there are, OPEN each message, and click the NOT SPAM button at the top right. You will have to do this several times before Yahoo Mail "learns" not to spam your Google mail.

OTHER WEBMAILS:
Other webmail sites may use similar blocking techniques to GMail and Yahoo. Search around in your settings, options, or preferences until you find the setting to show the pics!