The only reason you are allowed to modify a copyrighted image for personal use is because that's considered fair use. The fact that you found it on a website is

The only reason you are allowed to modify a copyrighted image for personal use is because that's considered fair use. The fact that you found it on a website is irrelevant. Copyright literally restricts using the material. If you buy a book, you cannot sell tickets to an event where you read it out loud.