If you want to use an image in your presentation, paper, or other project you can legally use an image that you have personally created (like a photo that you took).
Often you want to use an image that you cannot create. There are many resources for finding images for your project.
Many images that you find online cannot be legally used in a paper or presentation. You can use photos that are in the public domain (typically this means something created before 1924) or that have a BY (attribution) license through Creative Commons. If you use photos that you do not legally have permission to use, you will be violating copyright law.