When you find an image online, the best practice is to assume that its use is protected by copyright. Some creators (and more every day) are relaxing the protections provided by copyright and intentionally are releasing their work under open content licenses such as Creative Commons licenses.
If you're looking for images you can use in presentations, proposals, or projects, it's important to look specifically for images that are free to use. Generally these are shared with the express purpose of reuse and are labeled with an open license or some other special designation. The resources on this page are some good starting points for finding images you can use!
(For more information about understanding copyright and licensing, you can visit this page on our OER guide.