Is it necessary to have picture files saved in Finder and in iPhoto?

Hi!
I’m new to Mac and have a quick question about saving my picture files. When I first got the MacBook, I copied my huge number of picture files into Finder (I treated Finder like the “My Pictures” folder on Windows computers). So then a while later I wanted to use iPhoto so I imported all of the pictures from Finder to iPhoto.
The problem is, now my 500GB hard drive is nearly full! I think that when I imported the pictures to iPhoto I may have doubled all of my pictures – is this true? Should I delete all of my pictures in Finder and would they be safe in just iPhoto?
Thank you for your help! I’m so afraid of messing up my pictures!