Help with messenger app saving images to phone gallery.

Can someone tell me how or why my messenger app is transferring pictures to my gallery from very old messages, even when I delete the image from my gallery it will show back up. My phones gallery folder image count for messenger images changes frequently, and I notice that images that I sent via messenger months ago are now showing up in my phones gallery. I can delete them from my gallery but they will show back up a day or so later.