In order to do that, you need to link the picture instead of uploading it.

In order to link a picture, it needs to be stored on a server somewhere that is always connected to the Internet. When you have put your picture on such a server, then you use the little tree icon that appears on the HTML Editor toolbar, and enter the URL for the picture there. My ISP, and I suspect many others as well; provides 10 Megabytes of webserver space for all subscribers, as part of our subscription. I can upload the pictures there, and then link them.

Do not try to link pictures that you have stored on your hard drive. They will appear for you, but everyone else will just see a red X where the picture shows up when you view your own post.