If you have uploaded any images which are not showing up, the first thing to do is to set the correct file permissions for your files and databases.
To permanently resolve the problem, you will need to modify a file within Magento.
Important Note: This is a manual hack which will be overwritten during a Magento upgrade.
Open up the following file:-
lib/Varien/File/Uploader.php
Look for this line:-
chmod($destinationFile, 0640);
Replace it with this line:-
chmod($destinationFile, 0644);
Look for this line:-
chmod($destinationFile, 0750);
Replace it with this line:-
chmod($destinationFile, 0755);
Save the file and the next image uploaded should load properly.