This is one of the challenges of the medium. Pages must be crafted in such a way that they render similarly regardless of the display device or the particular browser's interpretation of web standards. Assumptions generally cannot be made. I see even professional sites designed for and probably tested on CRT monitor resolutions which then look abysmal on a widescreen LCD.

Setting size information on your images, use of tables for layout, and cascading style sheet features (CSS Tutorial, Starting with HTML + CSS) all can help persuade the browser to locate content where you want it. Direct editing of the HTML code may be required for complex layouts. At this point, you will probably find yourself fighting FrontPage over the HTML it generates (Comparison of HTML editors).