Is it possible to show an alternate image if the original source file is not found? I would like to achieve this only with css and html, no javascript (or jQuery and alike).
You can do this using the CSS background-image property of the img element, i.e.
img { background-image:url('default.png'); }
However, you have to give a width or height for this to work (when the img-src is not found):
img { background-image:url('default.png'); width:400px; }