Thanks a lot for the answer, very helpful as always.
I have one last question, I'm trying to force 'original' version of images to 800 x 600.
The following code retrieves the correct version but does not force original size to 800 x 600. I'm sure it's a syntax error again. Here's my code:
$t_object->getRepresentations(array('original', 'preview170', 'medium'),array('original'=>array('height'=>800,'width'=>600)));
Thanks again.
Sebastián