The application or the photo library? I would recommend leaving the application right where it is.
To move the library just select and drag the iPhoto Library to the external hard drive. When you next startup iPhoto hold down the Option key while starting iPhoto and you will be asked to choose the library you want to use.
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To move the library just select and drag the iPhoto Library to the external hard drive. When you next startup iPhoto hold down the Option key while starting iPhoto and you will be asked to choose the library you want to use.