When I needed dna picture images for a project last semester, I found that Wikimedia Commons had a great selection. They offer a variety of images, ranging from simple diagrams to detailed renderings, all available for free use. Just make sure to check the licensing to ensure it fits your project needs. Additionally, websites like Science Photo Library and the National Institutes of Health (NIH) Image Gallery are reputable sources for scientific imagery, including DNA structures. They might have what you're looking for as well.