The first possibility is that he really regards you as a friend. Maybe he found many of the same topics as you during his trip, especially in fitness. He also wants to make friends with you, that's all. Maybe you also have your own unique views on fitness, and he especially appreciates it. Then send some photos for your guidance. He may feel particularly compatible with you, but he doesn't want time and distance to pull your friendship apart, so he will show you what he likes and chat with you.
Of course, I think the first possibility is relatively small. I think he should just want to seduce you, so he will send you a fitness photo. After all, this is fitness, Zhang Chi. Why would he send such a photo to a girl he doesn't know very well? This must be an attempt. After all, a girl's words can rarely resist the temptation of muscles, so he first puts his own photos to brush his good feelings, and then adopts other ways to chase you. Maybe he fell in love with you at first sight during the trip, or he developed feelings after getting along for a few days, but maybe he didn't confess because he was embarrassed, and then after you broke up, he found that he couldn't forget you and was ready to pursue you. This is a very big possibility. After all, you two don't know each other very well He sends you fitness photos for no reason, every day. Obviously, he wants you to appreciate his muscles, so as to appreciate him. He has ulterior motives.
In either case, I think you need to talk to him and ask him what he means, otherwise he will still send you photos.