Looks like you've stumped the others.. :D
Anyway, as far as I can see, all you would need to do if put a series of wide images (all the same amount of pixels high and, preferably, wide) into /wp-content/themes/atahualpa/images/header By default, if there's more than one image in this folder it will rotate between them. If you then go to ATO>Style & configure LAYOUT and set a percentage in the first box of something like 75% this'll make the site fluid. I guess you have already done this. Then set the maximum width to the width of the 'wide' images you put in your header folder, or of the narrowest image (remember to put px after the width value). Set the minimum width to something like 400px so you don't get any site morphing when the window is shrunk by a lot.
I think this should work, sorry if it doesn't. :|