What is the meaning of the photo "Destroy This Mad Brute"?

"Destroy this Mad Brute" is a piece of World War I propaganda, originally produced as a lithograph by Harry Ryle Hopps, an American artist and illustrator. The piece was produced in 1917, the year in which the United States entered the war. World War I had been going on for several years by the time the United States joined. Woodrow Wilson (who was at first reluctant) approached Congress with a declaration of war in April of 2017, after Germany sank a British merchant ship called the Lusitania and an American steamer called the Housatonic.
The poster is a piece of propaganda insofar as it encourages young men to enlist in the US Army (who commissioned the piece). The poster shows a gorilla, who wears a helmet that reads "militarism" and carries a club that reads "kultur." The gorilla also carries a woman in its other arm. The trope of a gorilla carrying a club was popular in art across cultures before World War I (though it was greatly popularized with the release of the film King Kong in 1933). The woman whom the gorilla carries might be interpreted as Belgium (a neutral country against which Germany committed atrocities such as killing civilians, which was widely disseminated by contemporary British propaganda). Alternatively, the woman might be a personification of Europe or America. At any rate, she is playing the role of a damsel in distress, and cities are commonly depicted as women in the history of art and sculpture (beginning with "Roma" in classical sculpture).
Additionally, the woman works on an emotional level by inspiring prospective recruits to protect their wives. Finally, the piece includes a background of a damaged cityscape behind a large body of water, representing war-torn Europe sitting across the Atlantic ocean.
The practical purpose of the advertisement is to inspire men to enlist in the U.S. Army, which needed troops to engage in a full-scale war such as World War I had become. The means through which it achieves this purpose is by demonizing the enemy and conjuring fear in the viewer.