the reason why can't Parse epub cover image

I opened some epub files and found something interesting. A epub file usually contains a file called titlepage.xhtml. When I added "../" to the "cover.jpeg" like this 
` <body>
        <div>
            <img src="../cover.jpeg" alt="cover" style="height: 100%"/>
        </div>
    </body>`
the cover could parse and the image showed.But I don't know why .Please help me resolve the issue.