This was a question I had about a month ago when I couldn't find out how to upload a banner to my Minecraft Server.
Step 1: go to a banner making website or app
Step 2: create the banner that fits your server
Step 3: when you are done creating your banner right click the banner and click save image as
Step 4: save the banner in your folders
Step 5: upload the file to your server editing section on planet minecraft
Step 6: get more players :)

I hope this helps!