The legal concept of "Fair Use" (or "Uso Legítimo" in Spanish) often applies, particularly for descriptive or educational purposes. You can typically use images in a blog post to review an episode or discuss the show. However, using them for commercial purposes—such as on a product you intend to sell—is generally prohibited without explicit permission from the copyright holder. Furthermore, creating fan art is considered a derivative work. While you can share it online, you cannot sell it or claim the original character design as your own.