Different rules apply if you are using an image as part of a class assignment versus creating something you plan to publish.
Go to TheVisualCommunicationGuy's website for a larger view of Can I Use that Picture? infographic
Images on a general Google search are likely copyright protected, but there is a way to narrow down the results to usable pictures and graphics.
"Open access" availability does not mean that the images in these collections are in the public domain--rather, it refers to their accessibility for research and educational purposes. For copyright and use restrictions, locate and read the image use policy for each collection.