Using absolute positioning will position your I-Frame a set distance from the top of the screen and a set distance from the left. Note that the positioning is relative to the VIEWPORT, so depending what size monitor you're using, it will change relative to your image. What is positioned correctly on your screen may not be the same on another computer.
Still that doesn't explain why the I-Frame appeared to move when viewed on the same computer. I'm guessing that you have your Favorites pane open, and widening or narrowing this will have an impact on absolutely positioned elements.
Are you familiar with CSS? If so, I'm sure we can fix this but it would be helpful if we could see the style sheet.