you mean more like this at https://snailrider.com/snailtrails ?
Here is how I did it.
Right click on the header image you want to change. Select the “open image” option.
Now look at the directory path in the address bar of your browser. The last part is the name of the header or banner. Write that down on a piece of paper.
Now open your ftp program up and navigate to the folder where that image was stored. Rename that image, just incase you want to use it later.
Upload your new header or banner. You must rename the banner or header image to that of the original, and that should work for you.
So basically you are just replacing the image using all the same file names and paths, with your new picture.
Make sense?