If you see a broken image symbol (eg. "red X" in Internet Explorer), the first and most important thing is to obtain the error message inside the broken image. The error message may help you to identify the problem.
The following procedure assumes you are using Internet Explorer on Windows, but it also applies to other browsers and operating systems, just the exact mouse clicks may be different.
Right click on the broken image symbol, and select "Properties". You should see the URL of the broken image. Copy and paste the entire URL to your browser "Address" field, and press enter. Your browser will then access the image URL directly (instead of through an <IMG> tag) and will show the error message.
In some rare cases, even the above method will show a "red X". For these cases, you may use the following method to capture the server output to a file.
Create a web page with just one line <A HREF='full_url_of_image'>abc</A>, where 'full_url_of_image' refers to the image URL obtained above. Then open this web page on the browser. You should see a link "abc". Right click on the link, and select "Save Target As". You may then save the server output to a file.
Note that you should do the above quickly (within two minutes of the error), because if the error is related to session variables, the session may time out and you may be unable to obtain the real error message.
Once you get the file, check if it is a valid image file (try open it with a browser or any image viewing program). If it is not a valid image file, open the file with a text editor. You may see a lot of "garbage characters" as the file is supposed to be an image. Look carefully to see if you can find any text message inside. If you found any text message, this is what breaks the image. You can then determine which part of the system emitted the message and correct it.