You've not said which browser you're using but it's almost certainly Internet Explorer because this is a well-known problem with Microsoft's pathetic browser. Unfortunately, the fact that the problem is well-known doesn't mean there's a universal solution.
Try this:
(These instruction are based on IE6. If you've got IE7, things might be slightly different)
Go to Tools > Internet Options > Advanced.
Scroll down to the section under 'Multimedia'.
If there's no tick next to 'Show pictures', click to insert one.
Then click 'Apply' and 'OK'.
If there's already a tick next to 'Show Pictures', click to remove it. Click 'Apply'. Then click to remove it. Click 'Apply' and 'OK'.
If that doesn't work, and you're using IE6, upgrade to IE7 (either from the Microsoft website or by looking for the program on one of the 'free' disks which come with computer magazines).
If you're still getting nowhere, read this lot:
http://support.microsoft.com/kb/283807
Finally, when you've given up and reached the stage where you're sticking pins into statuettes of Bill Gates, download a decent browser which actually works:
http://www.mozilla.com/en-US/
Chris