Edit picture messages

Hi all, so when I started my server I put some image files like banners such as Welcome or the name of the text channel at the top of my server but they were pretty rough around the edges and since then the server has grown a bit and has more of a theme etc. so just wondering is there any way I can go back and change those images? I can’t find a way and seems like you can’t. I can’t even seem to edit normal messages but add a new image and maybe delete one or two messages so it goes back to the top. Any suggestion or ideas how I can go back and edit it to look a bit better? Only way I could see how would be to either start a new one and delete the old or delete all the messages and essentially start it again. Neither scenario is ideal though!