Custom Header won’t Repeat

You are targeting #header img(incorrect) and using repeat-x without any background-image property.

Use this CSS code:

#header {
  background: rgba(0, 0, 0, 0) url("http://playfulsstudio.com/wp-content/uploads/2015/05/cropped-playful-logo.jpg") repeat-x scroll 0 0;
}

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