The path to the image isn't right, the value of the src="" attribute.
Are you trying this on a server or locally? If on a server, are you sure you have uploaded the images? Do the paths in your code reflect where the images really are? Maybe you link to them on your hard drive or you have uploaded them to another directory than you meant to. Another possibility is CaSe. Most servers are Unix based and case sensitive. pic.jpg, PIC.JPG and pic.JPG are three different files on such a computer.
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