In Leopard or later...

When adding an attachment via a File Open window, scroll down in your sidebar to the Media section. You'll find Photos which gives you access to your Library structure.

You can also drag-and-drop a thumbnail icon from iPhoto to other apps to open the full image or attach it. You can drag-and-drop to the File Open window.

You can right-click a iPhoto thumbnail and select Show File to reveal the file in a Finder Window.

You can also right-click the iPhoto Library in the Finder and select Show Package Contents to browse the files.

Note, do not change anything regarding the files within the Library package via the Finder or you could cause yourself problems in iPhoto. It's best to just use iPhoto or the Media Sidebar to access your photos.