How to set pictures when posting links to facebook?

Hi, i always post links to pages on facebook and never really think about it when it comes to choose the image. So today i tried to share a page i made but facebbok only displayed the images directly on the code, and ignored all css, includes and xml image references,... I tried hardcoding an image on the index, with “display:none” (so it wouldnt change the page) but facebook ignored it as well… Does anybody know, or can link me to, some document or code where i can learn how to display a specific image when sharing a link on facebook?