that every time you change it, the replacement image is what will show for any and all your past posts. (The image does not actually reside with each post, but each post has the instruction to go get the image file on-tap and display it.)

All this means is that if you make a holiday related post now and someone views it this Christmas, it will not display the shamrock and instead will show your Christmas tree (or Santa with a ball cap) or whatever new image you put into the file then.

Jim