The most common reason for this is that the link pulling up the pages/pictures is addressed incorrectly.
A common way of incorrectly addressing the link is to build the site on the home computer where the pictures are kept in a directory of the hard drive. Then, when it is uploaded the links are still addressed to a directory of the hard drive. The easiest way of finding out where the link is addresses to is to right click on the empty box or the 404 field and then select "Properties". You will then be shown the address of the link. If this starts "c:/....." Then it is still addressed to a directory on your hard drive.
So obviously you need to change this to the address of the image on the web site. The other way that this can be addressed incorrectly is that either the directory or the page/image file in the address is spelled incorrectly or does not exist. It may sound very basic but the pages/images do have to be loaded onto the site and at the location you are specifying them to be at. It is always worth checking this as it is often the problem. So, for example, if you had a web site called www.abc.net and you are linking to a page file called "images.html" contained in a directory on the web site called \"pages\" then the address then the link would look like:
www.abc.net/pages/images.html The images are missing because the path to them is incorrect