To get this dialog, just click “Image” from “HTML→Miscellaneous” menu or from “HTML toolbar”.
Here you can choose an image to insert into current document. You can leave the default properties of image (its defined automatically) or you may determine your owns.
Note, to check the “Relative to Document” option, at first, you should save active document.

See also other dialogs: Preferences Find & Replace Send by Email Insert Table HyperLink Email Link Named Anchor Form Elements Insert special character Customize date/time format Formatting JavaScript Call JavaScript Function