How can I copy my pictures folder on MacBook Pro to my external hard drive?

After plugging in the external hard drive you'll see it mount. Depending on whether you've configured external drives to appear on the desktop, you'll either see its icon on the desktop or, you'll have to open a Finder window and you'll see it's icon in the sidebar to the left under Devices. Double click on the drive's icon to open a Finder window to the drive. Now open another window to your pictures folder and drag the pictures folder onto the external drive.

Do you want to copy the pictures or just move them to your external to save space on your Mac hard drive?

I hold down the Command key after I selected all of the images I want to move, then drag and drop them into the external. By holding down the command key that moves them from your Mac to your external drive, and allows you to recover that disk space back on your mac.