Just a simple question, why is pixel.gif usefull? And why should you use it or why not?
Pixel gifs (often called shim.gif) were used in older web designs (i.e. older than, say, 5 years ago) to provide a space in page or table layouts where a better understanding of robust CSS was lacking or if browser differences (typically between IE and Netscape) made life too complicated.
They should be considered a relic of the past and would not be seen in a professional CSS layout in today's web design. However it is still possible that older pages that haven't been maintained over the years will still have table-based page layouts with pixel shim images to provide some artificial spacing for columns.
In short, you should not use them unless an image is required for non-javascript statistics gathering for your site.