Hello STM 
I'm looking to run game installs on Facebook.
My affiliate networks don't provide creatives.
If I want to run Candy Crush, for example, am I allowed to use images I find on Google or on the Candy Crush Facebook page? Or would this be a copyright violation?
Also, could I take screenshots from game play I find on Youtube or other sources, and use those in my ads?
Lastly, are there any other sources where I could acquire images to use?
Thanks a bunch, great forum!
First of all I want to start by saying that whenever you choose to use images without having received explicit permission by the owner of the images to use them for advertising purposes, you run the risk of copyright violation/infringement.
So that would be the standard answer. 
However, in the real world, affiliate marketers will often just grab images from anywhere they can get them, and use them in ads without thinking twice about potentially infringing on copyright. I know I've done everything you mentioned in your post - ripped images from google images, the appstore/playstore, facebook pages, screenshots from youtube vids - and I haven't gotten into any sort of trouble yet. However, that doesn't mean I never will.
What I'm thinking is that if worse comes to worst, I get issued a C&D, I would just stop using the image then. But that's just me.
Of course, if you want to be 100% covered, consult a lawyer.
It may be a good idea to ask the advertiser (through your AM maybe) for permission to use images posted with their app on the appstore/playstore or images from their website. It would also be good to ask if they have images they could give you, even though they don't have ready-made banners available.
Amy
Vortex pretty much nailed it, that's how it works in the AM world 
On the positive note, the risk of getting into trouble is veeery low when you use their imagery to promote their own apps or products. So if you use pics from candy crush facebook page to promote the actual candy crush app, the risks are pretty much non existent.
On the other hand, using for example whatsapp logos and other graphics elements to promote the "whatsapp" upgrade offers that have nothing to do with the original whatsapp app, that's another story 
Cheers, thanks for the help.