Rynea
In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; the nature of the copyrighted work; the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the effect of the use upon the potential market for or value of the copyrighted work.
-- 17 USC Section 107 / Section 107 of the Copyright Act / Fair Use
A derivative work is transformative if it uses a source work in completely new or unexpected ways.
(source here)
I base this guide off of the covers that I make & my experience. I consider my works transformative, so I wrote this guide based off of that.
With that being said, I understand that not all people are going to go and make their covers transformative. They're likely going to just use a picture and call it a day, so I will add a warning for that. However, I would appreciate it if you thought about fair use before you said that "you are not even allowed to modify it without permission", since that is incorrect.
Also, I believe that the reason that this thread is stickied is completely different from the information about copyright. It was stickied because there are tens and tens of threads being made that ask the same questions about HOW to make a book cover; not the details of how legal book covers are. When I wrote that section, I was thinking more of the times where people ask me where I get my images from, and that would be my response for photos I use from English sites since I primarily go on Chinese sites to find the images I like (and others won't be able to access them since they don't understand the language). Therefore, I was not thinking much about copyright when I was writing that section, so although I'll admit that I was wrong to not include that information, that was the reason for why I did not include an in-depth explanation about copyright.