Not all that much to document:

1. Click the "tree" icon (Insert/edit image)
2. Enter URL of picture to load
3. Choose alignment (eg. "middle" to center on its own line, like the one below)
4. Drag to correct size (note: if your image is right-aligned, you have to use the handles on the left side)
5. Enjoy!

To adjust the parameters later, just select the image and click the "Insert/edit image" icon again.