Custom header

Hi, I am looking for getting a different (in fact, a "well cropped") image to use as header on each of my pages, without using the featured image linked to each page (bad cropped result on the header). I wanna keep the featured image of each page for miniature display but have a different sized/cropped image for the header of each page. How can I do this ? The plugin "Unique header" seems to be in conflict with the theme because it doesn't display the imported image. Please help.