Size of slider-picture changes in fullscreen

Hi, my HP (full frame) has 3 Featured-Slider pictures. It took me a certain time to get them same size (Promotion headline was hopping with each picture). Now it works fine but when I change my browser (safari) to fullscreen, one of the pictures is smaller than the others and the whole page is changing its position. What can I do to keep them equal? Sorry for my englisch - wwww.frenchchamberphilharmonic.com Thank you for your help! flp