You can add more complex plotlines and character developments to make it longer. Make the situations more challenging and darker to make it grimmer. And introduce unexpected twists and engaging subplots to make it more interesting.
Sure. There was a story about a group of miners in a small mining town. The mine they worked in was deep and dangerous. One day, there was a cave - in. Many miners were trapped underground. The conditions were dire. They had limited air, food, and water. As time passed, they started to realize that there might be no rescue. But among them was an old miner named Jack who had a lot of stories about the mine's history. He told tales of strange creatures that were said to lurk in the deepest parts of the mine. Some of the miners thought he was just trying to scare them, but as the days went by and they heard strange noises, they began to wonder. This situation of being trapped, the fear of the unknown, and the stories that were shared made it a longer, grimmer but really interesting story.
Imagine a lost traveler, Mark. He was sailing in the ocean when a violent storm hit. His boat was damaged, and he was adrift. As days passed, he saw no sign of land. He was running out of food and water. One day, he spotted a small island in the distance. When he reached it, he realized it was a very strange place. There were ruins of an ancient civilization. The air was filled with a sense of foreboding. He explored the ruins and found some strange artifacts. But as he did, he felt as if he was being watched. There were no animals or signs of fresh water on the island. Mark's struggle to survive, his exploration of the mysterious ruins, and the overall sense of isolation and danger made it a longer, grimmer but very interesting story.
Well, there are several places. Graphic design forums sometimes have users sharing such clipart. Also, specialized clipart libraries like iStockPhoto could be worth checking out. They usually have good quality stuff.
Young cartoons often focus on simple and fun themes, with bright colors and easy-to-follow stories. Old cartoons might have more complex themes and diverse art styles.