Interesting. I was having this problem trying to change the banner image on a project page a couple days ago. I could delete it, but when I tried to upload a new one, it would always just show me the old one. Much like the photo rotation problem that people have been having (me included), I just left it alone and it eventually (hours later) changed to the correct, new photo - just like how lately when you try to rotate an observation photo, it looks like nothing happens, but eventually (hours to days later), the photo rotates.
Related issues?