I cannot say this enough. Remember: for the most part, we work . Print is different than web. You will want you use large, high resolution photos. Here are some ways you can do that:

Google Image Advanced Search – Change size to large and, preferably, change usage rights to “free to use or share”
Flickr Advanced Search – Check “Only search within Creative Commons-licensed content”
MorgueFile – Free stock image site
freeimages.com – Name speaks for itself.
Stocksnap
Gratisography
Unsplash
Superfamous Images
New Old Stock
Magdeleine
Picography
Isorepublic
Raumrot

Now the company you are designing for may have many good quality photos that you might think could get the job the done. If you ever have the question, “How will this look when I print it out?” then the solution is simple: print it out. Look at it and examine it. Does it look pixelated or distorted? If not, you should be good to go.

Attribution

Unless an images is a CC0 license or Public Domain make sure you give attribution to the author! It's best if you give their full name unless it's a username, which it's best to say something like "flickr user insertusername"