Having the image prepared for use:
1. The image must be 512x256 pixels and be a PNG file.
2. Upload the image to a image sharing site such as Imgur.
3. Copy the link from Imgur for your image.
Uploading the image onto your server:
1. Go to you the page for your control panel.
2. Save and stop your server.
3. Go to Configuration files.
4. Click `Text Editor` for server.cfg.
5. Add the variable server.headerimage "image link here" with your image link replacing `image link here`.
6. Save the file by clicking the save button just about the text editor.
Restart your server and the header should now be your new image.