Headers and footers can be setup with a particular template.  New accounts come with one header and one footer already created.  To modify the header/footer associated with a template, edit the template, then click Advanced Settings.

To modify a header or footer

In the menu click Templates & Pages > Template Headers & Footers.

Headers maximum image height is 150 pixels by 1200 pixels wide.

Footers must be within the dimensions of 1200 pixels wide by 50 pixels high.

Page numbers in footers

Add in the following code to get page numbers

Page &p; or &P;