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Since I updated to Drupal 7.20, images created with Dynamic Background, are not visible.
There was an mportant change in image generation: drupal-7.20-release-notes:
... any code which programmatically generates a link to an image derivative without using the standard image_style_url() API function will no longer work correctly if the image does not already exist in the file system ...
Quick and dirty workaround: adding "$conf['image_allow_insecure_derivatives'] = TRUE;" in the settings.php
Find it there: http://drupal.org/node/1923936