editied to add support for ie6:
Try
<img style="height:725px;max-width:500px;width: expression(this.width > 500 ? 500: true);" id="img_DocPreview" src="Images/empty.jpg" />
This should set the height to 725px but prevent the width from exceeding 500px. The width expression works around ie6 and is ignored by other browsers.
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