SD Cards

Mount the SD card on the Desktop by inserting it into the SD card reader of your iMac. All iMacs that are newer than the mid 2009, 20" iMac9,1 have a SD card reader built in.

Export the pjhotos you want to put on the memory card to a folder on the Desktop.

Drag the folder onto the memory card's icon to copy them to the card.

But note that if you plan to display them on a camera, it might not be possible. I know Canon's are very finicky about displaying jpegs not created in-camera.