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!
"Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone." It works like Wikipedia in that everyone can edit, but all images found here are available for reuse via open content licenses or because they are in the public domain.
"CC Search is a tool that allows openly licensed and public domain works to be discovered and used by everyone...CC Search searches across more than 300 million images [and aggregates] results across multiple public repositories into a single catalog, and facilitates reuse through features like machine-generated tags and one-click attribution."
Use the Advanced Search or the Tools menu of the results page to limit search results to "Labeled for reuse" or "Labeled for reuse with modification" (screenshot below). Admittedly, their labeling isn't always perfect, so be sure to click through to the original source to save/download the image and record the necessary attribution and license.
Just don't forget that citation! At minimum, be sure to include the source of the images you use, but it's even better to include title, author, and license as well.