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.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.