President John F. Kennedy and First Lady Jacqueline Kennedy Attend Ballet Folklorico Performance, Mexico City, 1962. ST-C1-11-62. Used with permission, John F. Kennedy Presidential Library and Museum.
Just like document primary sources, primary source images are generated at the time of the event. Due to technologies available at the time, images can be paintings, hand-drawn maps, and photographs (note older photographs might not have not aged well) or a statue or other 3-D object. If the item is not something you can examine in person, check with your professor regarding use of a high-quality photograph.
Use only reputable sources for original, high-quality photographs or reproductions. A reputable source is an established organization with a solid scholarly reputation. The organization provides basic information such as artist/photographer/creator, date and location for context and usage permission or credit line. Reputable collections are generally universities, archives, museums, or legitimate government sites.
When in doubt, ask a librarian or your professor for help.
Keep in mind much of what you see has been removed from its original context. Photographs may be removed from a scrapbook, reproduced images may have been cropped or altered in some way. A reputable organization will note any alterations or damage that could affect the meaning.
In addition, you may find helpful notes or description to place the image in historical context. Do not use this information as a secondary source as it will not have gone through the rigor of peer-review, but do use it as a 'launch pad' to understanding your image, its importance, and application to your historical topic.
Using Google Images to Find Images
Google images is one way to locate historical images but has some challenges as well.
There is no way to limit the search to primary sources.
You will find historical and modern images mixed together from sources ranging from hobbyist sites (Pinterest) to scholarly museum collections.
What you see in Google Images may not be the entire original image. Clipping a piece out of a larger image alters the context and meaning.
Do NOT cite Google Images as your source. You must follow the trail to the original digitized image source.
Check for permissions. Finding it on the internet does not mean you have permission to use. Most institutions allow no-fee use for educational purposes such as student papers, but there are some that do not (Getty Images for one.) Others require you to request permission to use and will generally grant for student use.
Here is a video on following a 'breadcrumb trail' in a Google Image search to the original image.