$donen = preg_replace('|\[img\]([a-z0-9?&\\/\-_+.:,=#@;]+?)\[/img\]|si','<img src="\\1" alt="Resim Ekleme">',$donen);

$donen = preg_replace('|\[img\]([a-z0-9?&\\/\-_+.:,=#@;]+?)\[/img\]|si','<img onload="ResmiBoyutlandir(this)" src="\\1">',$donen);

<script language="javascript">
<!--
function ResmiBoyutlandir(img){
if(img.width>="500"){
img.width = "500";
}
}
//-->
</script>

$donen = preg_replace('|\[img\]([a-z0-9?&\\/\-_.:,=#@;]+?)\[/img\]|si','<img onload="ResmiBoyutlandir(this)" src="\\1" style="cursor: pointer;" onClick="javascript: window.open(\'\\1\', \'resimler\', \'scrollbars=yes, left=1, top=1, resizable=yes\');">',$donen);