Background image not comes in pdf

Dear all
I am faced to a print formatting problem (like: sales order)
I need to apply my own brand on print format,

  1. I put an image background for class named ‘print-format’ but background image does not come in pdf as I designed.
  2. But if I use letter head according to guide here, the style which I give for image, that is different from ERPNext view to pdf.

background image which I added

background image in pdf

If I use letter head, see the problems

letter head which I designed

letter head comes in pdf

please help someone how to solve this problem