Please don't add or move pictures in the iPhoto library in your pictures folder. If fact look but don't touch as far as the iPhoto library file goes

Add the pictures by dropping them into the open iPhoto app or iPhoto > File > Import to iPhoto.
In iPhoto > Double click photo to edit > crop plus any of the other options > Done.
Use System prefs > Desktop > iPhoto Albums to set as desktop.

For simple cropping and saving pictures that you don't want in iPhoto Preview plus a normal folder in Pictures (right click > New folder) works well..
S.