Currently it is not possible to resize contenteditable images live on the page without a workaround. This is a serious issue with javascript based WYSIWYG editors.
There is a lot of earlier discussion in bug 12550, but apparently no bug about resize handles filed yet. Marking this as blocking for now.
*** This bug has been marked as a duplicate of bug 7154 ***
(In reply to comment #1) > There is a lot of earlier discussion in bug 12550, but apparently no bug about resize handles filed yet. Marking this as blocking for now. I think you mean bug 12250.