1- Is this Fair Use of an image?
2- Did I break the law?
3- Did I remix or use a small enough portion of the original image to make this legal?
4- If I wanted to use this image in my portfolio or for commercial purposes, would I need to get permission?
What my understanding about this topic is that it is a very fine line between Fair Use and breaking the law. Fair Use means that an image or anything really is open to the public to use and appropriate into his or her own version of "art", whatever that may be. It is breaking the law when it hasn't been altered, Photoshoped, or changed completely from the original and used for commission. One can borrow imagery, but it has to be almost unrecognizable from the original to be safe.
My images from both of the Tweed examples have some that are safe and some that are considered 'breaking the law', because I didn't not alter it enough. I just changed the colors or moved things around which would be "stealing" (if I decide to sell it). Most of my images are safe because I incorporated my imagery into enough that one should tell where the appropriated image originated. The images that I do like enough to use in my portfolio would be safe where I wouldn't have to ask permission because the appropriated image isn't the main focus and is completely altered. I took in small portions in most of them that they would be fine to use. The ones that aren't I know because it is basically, entirely the same as the original.