http://www.work.com/creating-a-great-business-brochure-285
Says that hp.com has free templates.

For a 3-folder, I start with the oowriter tri-fold brochure template.
I Note the column widths, then change the page formats to set 0 margins
all around, noting the original margins (.5" H, .75" V).  Then fix
existing columns.

Do Edit / Columns, not Page / Columns.  Unclick the auto-sizing stuff.
Create 2 more columns.  Change the column and divider sizes as follows:

    orig-page-H-margin     orig-col-w             ...      ...      orig-p...
                        0"             orig-div-w     ....      0" 

BG image prep.  Make sure in Gimp <Image>/Image/Print Size has WxH of 11x8.5.
(Probably just resize with units of inches!).
X and Y res of 72 pix.in. looks fine, even though my printer does 600x600!

Under page settings, set the background image.