Images, songs, videos, other non-textual works are covered under intellectual property laws, even if they don't have a copyright symbol. If you're looking to use an image, make sure you either use public-domain work (no permission or attribution required) or look for material which is licensed for use (try searching for images licensed through Creative Commons).
However, for instructors and students, in general, images used in a classroom presentation, for a scholarly lecture, or in an unpublished assigned paper, fall under the concept of Fair Use or the TEACH ACT. Fair use is an exception to the exclusive rights granted by copyright. For further information, consult Circular 21 of the United States Copyright Office. So you don't necessarily need to ask for permission. However, you still need to provide an attribution for the image; technically, a entry for the image should go into your References list as well.
Keep in mind that if you want to present your image in a PowerPoint, it should be at least 72dpi, and about 1024x768 pixels.
U.S. Fish & Wildlife Service images
USA.gov image search. All federal materials are public domain.
Google Search (images). use limiter "site:gov"
Creative Commons Images search
Smithsonian Institution Open Access For the first time in its 174-year history, the Smithsonian Institution has released 2.8 million high-resolution images from across its collections onto an open access online platform for patrons to peruse and download free of charge. Featuring data and material from all 19 Smithsonian museums, nine research centers, libraries, archives, and the National Zoo, the new digital depot encourages the public to not just view its contents, but use, reuse, and transform them into just about anything they choose.
Your image source should be attributed with both in-text citation as well as a corresponding entry in your References list.
Use a Caption: A reader should not have to refer to the text to understand the image. Explanatory text should include title, owner/artist and where the image is stored. In APA you must provide a copyright attribution in addition to citing item when you reproduce it in the body of your work. See Publication Manual of the American Psychological Association, 6th edition, 2.12. p. 38. For educational projects, look to instructor's instruction for further guidance.
CITING images
In general, citing images can be complex. But don't fret; just do the best you can. Find photographer or artist's name; a date the photo was taken or when the image was created; where the original is stored; and where you found it (URL or publication information). Construct a citation as best as you can with that information.
Here's a quick guide from Cornell University.