how do i move my iphoto library to an external drive so that future photo downloads are done directly in the external drive?

Hello,

I opened my iphoto library in finder and hit command+c and then command+v to copy and paste it into my external hard drive. Now I see iphoto library in my external drive and when I click on it, iphoto opens. Now what do I do? I want to make sure that the next time I connect my camera to my computer, that I download the pictures directly to the external drive so that they don't take up space on my MAC. Currently I'm getting a message in iphoto when I try to download pictures that something like "low space on start-up disc". This is why I need to figure out how to download pictures directly to the external drive.

Thank you.