[shortcode]
<img src="http://www.site.com/myimage.jpg" />
<a href="http://www.blabla.com"><img src="http://www.site.com/myimage2.jpg" /></a>
(...)
[/shortcode]

<!--- shortcode code before input -->
<img src="files/myimagescript?http://www.site.com/myimage.jpg" />
    <a href="http://www.blabla.com"><img src="files/myimagescript?http://www.site.com/myimage2.jpg" /></a>
    (...)
<!-- shortcode code after input -->

function append_myimagescript($attr, $content){
    $pattern = '/src="([^"]*)"/i';
    $replacement = 'files/myimagescript?${1}';
    return preg_replace($pattern, $replacement, $content);
}

function bartag_func( $atts ) {
    $atts = shortcode_atts( array(
        'foo' => 'no foo',
        'baz' => 'default baz'
    ), $atts, 'bartag' );

    return "foo = {$atts['foo']}";
}
add_shortcode( 'bartag', 'bartag_func' );