Generally it's not a very good idea stylistically to centre images in the main body of the page, but it's OK in the right-hand column.
Click on the HTML gallery button on the editor's toolbar
Select centred-pic and click on the green tick button to insert it:
Make sure the image is selected:
Click on the image properties button.
This will open the Image properties window. Click on Browse Server.
Browse the directory tree in the Image window to select the image you want.