Quite simple. you have the wysiwyg editor embedded into the admin widget control panel area. then you simple generate the code, add images etc that you need for your sidebar widget, copy and paste the generated code into the text widget. viola! you would use the same wysiwyg editor that ships with wp. all you are doing is linking to it from the widgets area.
why hasnt this been done? u can edit a page easily but if you want to add html to a widget you are stuffed unless you are a coder.
Pete (all rights reserved! Lol)