To create a photo-manipulation, I use images from a variety of sources for reference. This is not an issue if the reference images are my own, or images I have acquired the rights to use. It is also not an issue in most cases, if someone sends me an image they have the rights to, and give me permission to use.
There are many, many wonderful photographers here on DeviantArt, who make available stock images for use in photo manipulations, etc. You scan through thousands of images, find the ones you like, follow their posted rules, and link back when it's time to share the final work in public. This works great, because it gives viewers the opportunity to see the before and after images, as well as giving credit and thanks to the artist supplying the stock images.
Then there are the images that are simply, "found on the internet". Many times I have found images that I am for one reason or another, inspired to manipulate in Photoshop. Many times, I am pleased with how they turn out, and I share them here on DeviantArt. Quite often, I have no idea who the original image belongs to. The person running the Tumblr, or whatever else kind of blog where I found it, usually has no idea where the original image came from either. I'm not interested in angering owners of these images, for using without their permission, so I don't post most of the works that I create from these unknown sources.
If you are the owner of any of these images I have used from sources I have not been able to find, please send me a note and let me know. I will be happy to provide a link back to the original reference images, give credit, or if you wish, remove it from DeviantArt.