Header (div) picture or graphic question!

[shepme]

Hello:
I have been studying HTML/CSS (web-design) for the past year. I have learned allot in that year, however there is one issue that I still have not found the answer to.
I create a div called header and set the margins to extend the entire screen. If I then view that "box" in a browser and keep hitting ctrl-"-' the box shrinks accordingly. This is where I am stumped. If I use html and tell the computer to put an image or a graphic that I have created inside of that box then shrink it as mentioned above (or change the screen resolution), the box shrinks accordingly but the picture or graphic starts to shrink much smaller that the box that it is in. If I enlarge the screen the picture or graphic will start to spill outside of the box. Most web-sites that have a header with a picture or graphic inside of the header ebb and flow along with the size of the box. Can someone please advise how this is done. I would truly appreciate any assistance.

[Wickham]

Give us the page url if it's online, or show us the code including any CSS file code.

Did you size the images in %? If you did, the browser may be reducing the size of the page by a certain % and then reducing the image by the same % again.

It doesn't happen when I tested one of my web pages because the images are not resized with a style, they just show the size that they were made with and shrink in the same proportaion as the whole page.