In papers submitted for publication, you cannot reproduce diagrams from others' work unless you have explicit permission from the author.
In student papers, it is allowable to include diagrams borrowed from others work. You must (a) provide that a full citation to the source of the diagram in your References section, and (b) included verbatim statement, next to the diagram, From from <insert author(s) name and publication year>. Reproduced without permission.
Please be aware that you cannot redraw images you found elsewhere unless you also give credit. In this case (for a redrawn image), you may (a) provide the full citation in your References section, and then (b) next to the drawing, include the verbatim statement Adapted from <insert author(s) name and publication year>.