How can you replace the photos on your iMac?
If your Mac has crashed and you have lost all data then unless you made a backup of all the photos they cannot be replaced. If you used Time Machine and your iMac was connected to an external hard drive then all your photos are on that external hard drive. Simply open Time Machine and restore all the photos to iPhoto.
Where in the iMac do you insert the photo memory card from your galaxy samsung cell phone or how do I download photos from the cell phone to the iMac?
There is no card reader on the iMac. If your cell phone has Bluetooth you can download the photos via Bluetooth. Make sure the Bluetooth is turned on your iMac. To do this open system preferences then click Bluetooth. Make sure it is turned on and discoverable. Also make sure your Bluetooth is turned on your cell phone. You can now send the photos from your cell phone to your iMac via Bluetooth.
Plug the USB drive into your iMac, then open the file folder to view the files. Also open the folder that has the images you would like to transfer. Click and drag from the computer folder to the USB drive folder. If you'd like to leave a copy on your iMac, just right-click and select copy, then paste it into the USB drive folder.