watermark_form_alter won't execute properly on node/add/image if Drupal is installed in a subdirectory

watermark_form_alter() runs when we're adding a new image. However the test to see if we're adding an image is too restrictive -- it doesn't consider the case where Drupal is installed in a subdirectory.

Patch attached to test that $form['action'] *ends* in '/node/add/image' as opposed to explicitly equalling it. When Drupal's installed in a subdirectory, $form['action'] will equal something like '/mysubfolder/node/add/image'.