can I point my Pictures folder to a folder in an external hard drive?
As Rabskatran pointed out, you can move your iPhoto Library if iPhoto is your primary photo manager. Or Aperture, it doesn't matter in this case.

Also, before we start, I would suggest you NOT to move your Pictures folder as it might mess with the structure of the system.

Anyway, here are the steps:


Close iPhoto if open,
Go to your Pictures folder,
Move that folder named as iPhoto Library to your external disk,
Make sure that the transfer is FULLY finished,
Open iPhoto while holding down the alt key on your keyboard,
From the pop-up Finder browser, choose your iPhoto Library on your external disk.


This way you transferred your iPhoto library to your external disk and made iPhoto to directly open up with your photo libraries new place.