In the past, whenever I used someone elseās creative work, I always made sure to credit the artist by listing the website, their name, and the title of the artwork. I remember learning about the Shepard Fairey Copyright Case in a class, especially the famous āHopeā poster of Barack Obama. My opinion is that I can see both sides of the issue. While Shepard Fairey did alter the photo, the original photographer was not credited, which led to the problem.
From an ethical point of view, no matter how much someone changes another personās work, they should always give credit to the original creator. Giving credit is also a good way to protect yourself and avoid potential legal or ethical issues.