I worked out my question now so thought I would post it if someone has my problem
My code would be like this
<img src="[+fietgall1:getpath+]/.thumb_[+fietgall1:getbasename+]"/>
I use two snippets
phx:getpath
<?php
return dirname($output);
?>
phx:getbasename
<?php
return basename($output);
?>