iPhoto stores photos in its own file structure within the iPhoto Library. You can change this option within the iPhoto prefs so that they remain where you put them; that makes them more easily accessible for some.
Assuming all your photos are within the iPhoto Library, there are two ways to get at them.
First, and easiest, is to open iPhoto. Select the photo and go File>Reveal in Finder (you probably want Original when given the choice). That takes you within the iPhoto Library folder structure, and you can do a control-click and open with Preview, or drop it on Preview. Once inside, you can navigate around to other folders, but you may find them difficult to find.
Another way is to go to Pictures/iPhoto Library and do a control click to get the Show Package Contents item, which when selected gets you inside. I have set it up so I can automatically get in there by using a symlink; I'm not sure an alias would work.
If this is a pain, then I recommend you store the photos outside of iPhoto's Library.
Rob