Hi!
I was asked to create something for a wall in a new restaurant that measures 112" by 70.5". It will be an Italian place and the owner has some photos from Venice he would like to use. I will probably be doing some effects to the photo(s) such as making most of the photo black and white while keeping one colour present. My question is how large/much resolution do these photos need in order to be able to print at this size and not look pixelated or blurry. The photos are 2848 x 2136. They are also in RGB format and 72 ppi. Can they be converted to CMYK/300ppi with no problems? Thanks for the help!
Well, to answer your question simply, they’ve gotta be larger than you have them right now 
It shouldn’t be hard to make your image 300 dpi, but that will probably be kind of bad. See, right now, your images are 39.6" by 29.7" Which is, of course, not large enough for the size that you want. You’re right in that you want the images at 300 dpi, but if you do convert it, you’ll have a print that’s only 9.49" by 7.12"
You’ll really need to take a better picture w/ a better camera - are you a professional photographer? Because you’d probably need a professional and a professional printer to do this job.
~TehYoyo
It would depend on distance that the people are viewing the image from - if it it’s 1 to 2 metres away you could probably push it down to 72 dpi (most professionals wouldn’t print a photographic image this large at 300dpi). But you’re many pixels short of that even - and you can’t just invent new pixels (maybe a lower resolution would work but you’d have to test it).
I would try getting larger resolution photos but if these are the images you have to use you might be able to get away with it. I would try scaling them to full size than print on some small pages without scaling them down as a test to see how it looks. But you’ll never have something that looks crisp up close but you might need something that clear - considering you need to be a few metres away to see the image in it entirety.
Just remember to do tests on small prints, stand back and test how it looks on you wall at home before you print the entire thing.
**you might not need something that clear
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. 
Thanks for the help everyone, sorry for the delayed response - I wasn’t getting e-mail notifications for the replies! I understand that when the picture is being viewed from a distance the dpi doesn’t need to be high, but the restaurant owner wants to have coffee tables right up against that wall, so while most people will be viewing it from a distance, the people sitting at said table will be very close to it. Perhaps he’ll need to rethink table placement? Thanks again for the help!
If he wants people right up against them, then you’re definitely going to need to increase the DPI, but that will also increase the printing costs (just a warning). You can increase the DPI slowly through photoshop - I’ve found doing it in small increment helps (ie. going 72 -> 96 -> 144, etc. instead of 72 -> 300), but I’ve never had much luck increasing them that much AND resizing the image - pixellation’s going to occur. CMYK vs RBG doesn’t really come into play as you’re only dealing with B&W with a spot color.
For a space that large, the collage idea is actually a good one. There are even plugins which can help with that, and could be a creative way to spice up the appearance - might be worth a shot. Far away it’ll look like one image, but as you move closer, the customer will see multiple images. Without the super high res images, this might even be easier, because otherwise the blurriness of the photos will be disrupting to the customer.
Thanks!
Very interesting exchange of ideas. It’s a bit tough sorting through what are the facts and what are the assumptions, but all of the comments made are pretty helpful in understanding more about printers and printing.