The URL in the background attribute must be relative to your web server or a absolute path to some files on server.
Your page: http:/ /www.mydomain.com/myDir/helloworld.html
Body: <body background="Dock.jpg">
It looks for: http:/ /www.mydomain.com/myDir/Dock.jpg
Body: <body background="images/Dock.jpg">
It looks for: http:/ /www.mydomain.com/myDir/images/Dock.jpg
Body: <body background="/images/Dock.jpg">
It looks for: http:/ /www.mydomain.com/images/Dock.jpg
Body: <body background="http://www.anotherserver.com/Dock.jpg">
It looks for: http:/ /www.anotherserver.com/Dock.jpg
And remember, the URL in the 'background' attribute will be the URL that the client will use to load the pic. Say, if I am viewing your page, my IE actually downloads the HTML to my computer, and then IE will render the page locally. When it sees "C:\Users\Public\Pictures\Sample Pictures\Dock.jpg" (and the valid URL should probably be "file:///C:/Users/Public/Pictures/Sample Pictures/Dock.jpg"), it will look for Dock.jp in my local hard drive, and of course, the file cannot be found.
