Is this just for you on one computer or is it for a company?

If just for you, create a template to hold the information for your headers and footers including images. Then select the text (and logo) for each header and footer and save it as an AutoText entry (Building Block) in that template (not in normal.dotm). Save the template as a global template and then use AutoText fields in your letter templates to insert the information into your letter template headers and footers.

When you need to change the address or logo or other info, change it in the AutoText entries and it will be pulled into other templates.

This is one way. There are others.

For an example, download the Letterhead System at http://addbalance.com/word/download.htm#AddIns.