Where are photos imported via iPhoto stored on disk? I imported some photos from iPhoto and want to attach them to an email message after exporting them.

However, Pictures -> iPhoto Library is not a folder. Where is the actual folder that these photos are stored in?
iPhoto Library → Right-Click → Show Package Contents
You have to search for your pictures. The order is not logical - i.e. sorted by events.
It's easier to choose the following options if you send an email:

In iPhoto go to Share → Email or ...
...export your pictures so that you can attach them manually File → Export

Why is the iPhoto Library stored in a package?
The reason is actually quite simple: People break things. When you accidentally mess around with the pictures by unintentionally moving them - you break the library. Photo Booth has its own package for the same reason.
However, if you want to have control over the hierarchy of your picture folders in Finder, you should not import pictures into iPhoto. The setting is located at iPhoto Preferences → Advanced → Importing.