It also depends on your browser settings or other things:
-The link to the picture on the webpage is broken(nothing you can do about this).
-Your computer doesn't have enough disk space to store the temporary files it needs to show the pictures(Check if you have enough disk space available in My Computer or the program used to display hard drive space on your current OS).
-Your browser is set to block images(The fix to this depends on your browser).
-It can also happen with some firewalls if they are configured to block certain content from certain sites(Also depends on your firewall).
Things you can try:
-If it's a Windows OS try to check Windows Update if you have the latest updates installed.
-Resetting your browser settings.
-Clear the cache like Whisper suggested.
-See whether pictures are available on another browser.
-Enable scripting, ActiveX controls, cookies, java etc.
-If you are seeing an image with an X on it. Install or update Adobe Flash Player(If you haven't already)
If your issue still remains unresolved please post more details about your issue what browser you are using, OS and how it's appearing to you and put a link to the image to test if the link is just broken or the site the image is on is just temporarily down.
