It really depends. Remember you can collage the photos.
Brace yourself this is going to get convoluted.
The RESOLUTION (DPI) will depend chiefly on your media and output method. Assuming we are going for print, you will need to consider the LPI of the device and how many TONES you need to get so that your image doesnt look "cartoony" or posterized. What I mean by this is if you have a devise whose LPI is adjustable you could use lower resolution, at the cost of tonality.
Also remember , pixelization is a visual illusion. Everything is pixelized if you look close enough. If the banner is 9' x6' how far is the viewing audience seeing it from? Do you think I really send a 300+dpi file for those Billboard ads you see near O'Hare?
ANOTHER TRICK to consider is collaging and PIXEL DIMENSION. For the sake of math let say you wanted to be able to print at 50dpi and that you final output was 90" x 70" .. this means 9000 px X 3000 px total area. You can collage the provided graphics (and use empty space) to fill that entire area. Yes , this will limit your layout options greatly, as you can't scale images with this method w/o ruing the effect but it will technically allow you to use the images provided images.
Hope this helps.