Primary source material is essential to the historian but they are not necessarily easily identified in the vast amounts of information available.
A good definition from the Research Users Services Association's helpful guide Primary Sources on the Web: Finding, Evaluating, Using states "Primary sources are the evidence of history, original records or objects created by participants or observers at the time historical events occurred..." [emphasis mine]
There is leeway in this definition as materials created well after the event such as oral histories or memoirs will also be considered primary source since the person who experienced the event is the one recording the oral history or writing the memoir. Just keep in mind, that the longer it has been since the event, memories become increasingly selective.
Even with primary source material, ask yourself if what you have is complete or biased in any way. If you are looking at a set of letters or a published memoir, consider how the editor or compiler made decisions about what to include. It may be certain materials were not included to portray a person or event in a certain light, material was considered irrelevant, or new evidence has come to light since the publication of the source you found.
Primary source material may be found within what appears at first glance to be a secondary source. Sometimes a secondary source item will look like a primary source. When in doubt, ask for help!
Searches including the term 'primary' will usually return an unsatisfactory result set. This is because it is actually difficult to label an item as primary--what a primary document is varies with the need of the researcher and the situation in which the item was created.
A photograph from 1911 of women soldiers in the Mexican Revolution is a primary source since the photograph was taken during the Revolution.
A woodcut of Adelitas (women revolutionaries) created in 2010, while it may look old, is not primary and should not be used.
The only time the woodcut could be used as a primary source would be if you were writing a paper on the modern representation of women who fought during that war. This might be a paper on how memory alters or romanticizes the past and as that is not the point of this project in this class, do not use it.