As littledynamite8 has said, you need to upload your image to your server. You should see your image when you type the address pointing to the image like so: https://your_server/image.jpg
Once you make sure that your browser shows the image from your server, make a reference to it in the blog, by clicking on the green thingie in the editor which will open up a dialogue box – Insert /Edit image.
Enter the URL of the image here. Add a description about the image and you can set the alignment here. Once you are done adding this, click “Insert” and you are done.