I was on a headache trying to test BG Module. Sometimes working fine, sometimes not.
Now I can see it was due to the fact I use a CDN service. When CDN server (CloudFront from Amazon AWS) receives a request for a certain image file for the first time, it tries to get the file from Drupal Installation. But on the very first time the image is requested, Drupal (or Apache2) sends an error (file does not exist) as response, instead of creating (resizing) the file for the first time.
I am surprised there is no single line about CDN issues inside five hundreds, hehehe.
Does someone have any idea about why this could be happening?