What are the implicit and explicit messages in the veteran's day poster?  

This poster uses several devices in order to convey a message. Some of these elements are implicit, while some other ones are explicit.

Among the implicit messages, the poster links Veterans Day with the United States. We know this because of the flag. We also see the sun shining behind the flag, which conveys an idea of glory and success. The image is positioned in a way that implies that the flag is high in the sky, strengthening this effect.

Among the explicit messages, we learn that Veterans Day is intended to honour all those people who served during the war. We also learn the name and the date of the celebration, and the institutions who are sponsoring the announcement.

the author motivation is to show the power of animal-human relationships.