There is no shortage of military artwork available. That being said, your request is very general, and I'm not entirely sure what you mean by indicating a picture's theme.
The best approach might be to figure out more specifically which part of the 1900's you wish to depict. For example, pictures associated with World War I will likely be very different from pictures associated with the Gulf War of the early 1990's or the war in Afghanistan.
I have provided a link to an article that offers an overview of military history in artwork. My best suggestion is to read the part of the article that covers the twentieth century, then perhaps peruse the examples of artwork that follow the article to determine if any of those pieces work for your cover design.
Obviously, you want the historical part of your cover to be accurate and meet the academic specifications of the assignment yet also be attractive and aesthetically pleasing. After all, the purpose of a book cover is to entice viewers to purchase the book!
By getting a sense of how military art changed during the 1900's, and by taking a look at several possibilities before making your decision, you can locate something that fits the overall scheme of your book cover as well as meets the requirements of the assignment.