AddHtml is for multi-styled text.
AddImageHtml is for true HTML/CSS web pages.
AddHtml is used for multistyled text. It's very fast. However it doesn't support HTML - it supports an HTML-like syntax - it supports HTML styled text. The key tags it doesn't currently support are img and table. It doesn't support CSS. It is designed principally for multi-styled text using an HTML-like syntax.
AddImageHtml is used for full HTML/CSS rendering. It's rather more sophisticated and complex but not as fast. It supports all tags including img and table. It supports CSS. It supports JavaScript. It supports everything you get in real-world web pages. You can use either the MSHTML engine for IE-like behavior and display or you can use our Gecko engine for Firefox-like behavior and display.