Be sure that your images are in the same folder as your HTML documents, and you can then use relative paths with the WebBrowser control. Don't use the Document.Text property for navigation. Use the WebBrowser.Navigate() method. This way the WebBrowser "knows" that the document and the images are in the same folder. Setting the Document.Text property does not tell the WebBrowser where your files are located.
From my experience with earlier versions of Word, it creates terrible HTML. You'd likely be better off to write your own HTML if you're comfortable with it, or visit download.com
] and find a different WYSIWYG editor.