Can you determine whether the photographer and the publisher have an agenda to communicate by publishing a particular photo from Vietnam War?
Although you have not actually mentioned which publisher and which book you are referring to, there are general principles that can help with the question.
First "agenda" is simply a Latin word meaning "things which must be done". Although there is a common popular misuse of the word, which implies that simply having a purpose is somehow nefarious and morally bad, that is not necessarily the case. All human actions are done with some purpose in mind; whether that purpose is bad or good depends on the specific purpose, not merely on whether some purpose exists.
A commercial publisher is trying to sell books. Authors of books select photos to vividly illustrate what they see as the salient points of a particular section of text; publishers approve selections on a basis of whether the cost of rights for the illustration is within the books's budget, whether the illustration is of sufficient quality to reproduce well, and whether the illustration is vivid and interesting enough to make a contribution to the book's sales figures. Thus a picture was probably chosen to illustrate what an author considered an important aspect of the war to convey to readers.
By nature, the vividness of visual imagery can lead viewers to react emotionally to the subject matter; readers can choose, however, to read text carefully for a more balanced and rational understanding.