Please help - changing YouTube thumbnail to default image

PHP
1

I want to change or replace every youtube video code from my post

<object type="application/x-shockwave-flash" width="425" height="344" data="http://www.youtube.com/v/luW4f2glk0Q">
	<param name="movie" value="http://www.youtube.com/v/luW4f2glk0Q" />
	<param name="wmode" value="direct" />
</object>

to something like this
‘http://i4.ytimg.com/vi/‘.$str.’/default.jpg’;

please anyone help… I know only little about html and css…

Thanks

2