What type of source is a photograph in a textbook considered to be?

Tertiary sources are sources that interpret, analyze, or summarize primary and secondary sources. They often provide overviews of topics and can be a good place to start research. In the case of a photograph in a textbook, the photograph itself is the primary source. The textbook authors have selected the photograph and placed it in the textbook in order to illustrate a point or provide evidence to support a claim. Therefore, the photograph in the textbook is a tertiary source because it is being used to interpret and analyze a primary source.