Make sure that your resource folder is listed as a source folder in your project settings. Also, make sure that the resource folder is set to export when you build your jar.
You can add a .zip extension to your jar file then open it up to ensure that your resources are included and at the expected location.
I always use absolute paths like this:
InputStream input = this.getClass().getResourceAsStream("/image.gif");
When you use absolute paths, “https://stackoverflow.com/” is the root folder in the jar file, not the root folder of the host machine.