How can I change the background and header of an e-newsletter template?

Hi, just wanted to ask if it is possible to remove the header/background files and replace them with files of my own?

I did this with some success and on the preview of my newsletter, everything shows up perfectly, with my new images in place.

However, when I send the email and open it, I see no background or header, just the text of the email.

Could you give me basic directions on how to proceed? Thank-you!