While the open dialog box is fine for adding documents that you can access through the Finder, you can't access your iPhoto pics through the Finder.
The left pane of a Finder window as well as Open and Save dialog boxes show your Favorites and your Devices. An Open dialog box also has a MEDIA section at the very bottom (scroll way down!) which gives direct access to your iTunes music, iMovie projects and best of all, direct access you to your iPhoto Library.
By direct access, I mean that you can access all of your Events, Faces, Photos and Albums, just as if you were in iPhoto itself. Select the pic you want to attach to your email and click the Choose button.
Don't forget that you can Command-click to select multiple pics to attach. Whether this works or not is dependent on the browser mail app you are using. Yahoo! Mail only attached the first pic I selected, but Gmail attached them all. Your results will vary on other Mail programs.
As easy as it is to attach pics this way to emails, the fact that the MEDIA section only shows in Open dialogs and that it is hidden at the bottom of the list on the left pane makes it very easy to forget about this great feature. I admit I've remembered and forgotten about this several times over the years.
Even though the focus of this tip has been attaching pics to browser-based emails, you can access the MEDIA section of the Open dialog anywhere.