Really NOOB question about iPhoto

I know this is probably a really dumb question but it's something I still haven't figured out since I switched over to mac about 9 months ago. I was wanting to change my screen saver to a slideshow of pictures and I found the "Source" button which brings up the directory where it wants me to select the folder with all the pictures in it. Ok, all of my personal photos were uploaded into iPhoto I can't seem to find out where they are. Sometimes when I open "Finder" there is a section that says "Media" and my iPhoto pictures are there in the photos section. Most of the time that **** "Media" tab isn't there and I don't know where my pictures are stored. I hope I didn't confuse anyone. Thanks in advance for your help!

in Mountain Lion try this.

Preferences > Desktop and Screensaver > Screensaver . Select one of the slideshows and change the source to photo library.
Hope this is what you want