Float an image in Design mode = On

Question

I'm using a WYSIWYG editor. It's nice, but it can't float images, so the text starts at the bottom of the pictures.

I'd like to implement a float left / right function. So i'm reading about Design mode = On

I followed this good tutorial: http://www.quirksmode.org/dom/range_intro.html

And now i can make an alert with text (for every browser). Not with html though. I get an error when trying to create a range. In firefox: Parameter is not an object" code: "1003

I don't know if ranges are the way. All i need is to float images. Any clue?