Moving photo from camera to iMac

Sometimes a few years go by between picture taking, so it's always a struggle for me. Anyway, I just took some pictures and then put the card into the slot in the back of my iMac. Here's where I got lost. I had thought that the pictures were supposed to automatically appear on my computer screen, but nothing has happened. Nothing changed on my compute screen when I inserted the card. Is there a step I missed?