That typically happens when you have the WRONG path to the image and/or you have misspelled the image name. Also, be aware that some webservers are case-sensitive. So if your image name is "logo.gif", then if you code "logo.GIF", it will not work.
So, your first task is to figure out the full path to your image. Type it directly in the browser's address:
http://www.yoursite.com/Images/logo.gif
If it loads, then you have the right path and you should be able to use that path in your image - <img src="http://www.yoursite.com/Images/logo.gif">
Also, if you are viewing your page via http, but your code uses the file protocol - <img src="file:///..."> then it will not work. You must reference the image using http, so you will need to upload the images to webserver.