You may expect a browser to do nothing when it encounters an empty image src. However, it is not the case in most browsers. IE makes a request to the directory in which the page is located; Safari, Chrome, Firefox 3 and earlier make a request to the actual page itself. This behavior could possibly corrupt user data, waste server computing cycles generating a page that will never be viewed, and in the worst case, cripple your servers by sending a large amount of unexpected traffic.
YSlow recommends:
Image with empty string src attribute occurs more than one will expect. It appears in two form:
<img src="">
var img = new Image();
img.src = "";
Both forms cause the same effect: browser makes another request to your server.
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