Just as written works are protected by the laws of copyright, so are photographs. To ensure that you aren't guilty of plagiarism, make sure that you cite the source of the photos you use. The Chicago citation style requires the use of a "credit line" placed beneath the photograph which acknowledges the source.(see Chicago Style Manual section 3.28 - 3.31)
Many of the photos you find online will have specific requirements as to how you must acknowledge them: always check the copyright or user restrictions statement identified with the photograph or the website.
For a fulsome discussion on this topic see:
Images can often provide information that can only be communicated and analyzed in visual terms.The photographic record is large: it includes images produced by anonymous or itinerant photographers, family snapshots, posed studio photographs, images taken by government or industry photographers. These photos were taken for a variety of reasons - to entertain, record an event, or express a creative urge. What the photographer chooses to produce is a reflection of what he or she thinks is most important to include and is a direct reflection of the attitudes and cultural norms of the day.
And remember - even though cameras today are ubiquitous to our everyday life - the first working models were only invented in the 1820s. They came into more common use later in the nineteenth century but were slow, complicated, and cumbersome. Development of photos required more time and an expert's touch. The amount of effort involved virtually guaranteed that the subject was chosen with deliberation and very little spontaneity. It was only in the twentieth century that advances in technology made the camera available to most people, and photos became more candid.
(caption: a photographer using a bellows style, wet plate camera with a Petzval style lens,circa 1865-1870) (source: antiquecameras.net)
For more information check the library catalogue:
Increasingly, historical photos are becoming easier to come by. Archives, museums, governments, and historical associations continue to digitize images and most are available online at no cost.
For some authenticated sources of historical images check under the "Websites" tab to the left, and select the geographical area you require.
Also, you may want to search some of the databases the Library subscribes to for photos:
