What do you mean by "target folder"? Where iPhoto stores the imported image files?
iPhoto does not have such a folder - it is a database system and manages the imported images and adjusted image versions inside an iPhoto Library. By default this library is located in your "Pictures" folder and called "iPhoto Library.photolibrary.
To import images into iPhoto use the "File > Import" command from iPhoto's main menu bar, and to export or share the images from iPhoto use "File > Eport" or "Share".
You also can access your iPhoto Library from the Media Browser in many applications, so that you not necessarily need to open iPhoto to use the images.
Post back, if that was not your question
There are to ways for iPhoto to handle your pictures: Managed (default) or referenced library.
For details see https://discussions.apple.com/thread/3313476
Short: managed mode takes care of your pictures by taking a copy that will be stored in the iPhoto-Library. Referenced mode is what you want. Only the path is kept by iPhoto. Pictures stay where they are.
Deselect "Preferences" "Copy files to Library" (or similar to this). Done!
It is STRONGLY recommended that you do not use a referenced library - doing so makes importing amd deleting photos more difficult and makes upgrading or replacing defective hardware very mush more difficult - virtually impossible
There is no reason to use a referenced library and doing so will cause problems over time
For one of many examples of the disaster or using a referenced library see the post just after yours - https://discussions.apple.com/thread/4473980?tstart=0
As to not wanting your photos on your computer's hard drive - simply move the iPhoto library to the external drive and use a managed library on that drive -
Moving the iPhoto library is safe and simple - quit iPhoto and drag the iPhoto library intact as a single entity to the external drive - depress the option key and launch iPhoto using the "select library" option to point to the new location on the external drive - fully test it and then trash the old library on the internal drive (test one more time prior to emptying the trash)
And be sure that the External drive is formatted Mac OS extended (journaled) (iPhoto does not work with drives with other formats) and that it is always available prior to launching iPhoto
And backup soon and often - having your iPhoto library on an external drive is not a backup and if you are using Time Machine you need to check and be sure that TM is backing up your external drive