Adding a photo to iPhoto by right clicking in the Finder
You can create a custom service for Finder using Automator.app. 
Open Automator.app located in /Applications/ folder by default.  When prompted to choose a document type, choose "Service".



On the top you will see a a box with drop downs.  Select "image files" for the drop down box "Service receives selected" and choose "Finder" for the in.



Then on the left hand side with the "Actions" tab selected choose "Photos" then drag "Import Files into iPhoto" into the workflow area to the right.



Configure the settings to where the photo will import into iPhoto.  The defaults are Existing album and Events.

Save your workflow and give it a name that will make it obvisous when you choose Services in the Right click menu.  (Ex.  "Import photo to iPhoto")



Services you create from Automator are saved in the path: ~/Library/Services/