Transform: Image {file} cannot be found with Why do i get an error

throw new Exception(Craft::t('Image “{file}” cannot be found.', array('file' => $file->filename)));

for 

{% set myImage = entry.pr_foto.first %}
{% set params = { width: 300, height: 200 } %}
&lt;img  src="{{ myImage.getUrl(params) }}" height="{{ myImage.getHeight(params) }}" width="{{ myImage.getWidth(params) }}" alt=""&gt;


while

{% set myImage = entry.pr_foto.first %}
&lt;img src="{{ myImage.url }}"&gt;


shows the image?

The filesystem path for photos is uploads/photos/. The upload works fine in the admin. It's only the transform.

Full trace error:
As i was working on a localised website I had to use the full path,
in my case on MAMP PRO that was:
/Users/davedriesmans/Documents/Sites/website.dev/public/uploads/photos/

guess i'll have to change then when going into production

use environmentVariables like Mats Mikkel Rummelhoff suggests