IME, the most frequent cause is that the server on which the picture is stored is down, too busy, or otherwise not responding to your request for the picture - and therefore nothing to do with you. Going back to the page later can sometimes magically produce the picture that wouldn't show earlier.
It is possible that your browser is set not to show pictures automatically, but if you are seeing some pictures but not others this is unlikely to be the case. You can check by going to Tools -> Internet Options, clicking on the "Advanced" tab and scrolling down to Multimedia. The box for "Show Pictures" should be checked. (This is where this option is located in IE 6.)
You can also sometimes get a picture to load manually by right clicking in the space where the picture should be, and left click on "Show Picture" in the menu that then appears. But if the server is down, this won't do anything.
Just occasionally you will come across a server which won't allow pictures to be loaded like this (can't remember the technical term for it). If all else fails, try the following. Right click in the space where the picture should be, and left click on "Properties". A box will come up which will give you the address (URL) of the picture. Select and copy this. Open a new browser window (File -> New -> Window). Paste the URL of the picture into the address bar of the new window and press enter. The picture may then load. If it does, leave that window open, go back to the original window and click "Refresh". That will often then give you the picture in its original context on the page.
Hope this helps - at least until the experts arrive.