I'll assume that the files you are looking to export (at least on your internal drive) are all in your home directory.
1) - Open a finder window and navigate to your home directory
2) - Above where it says "Search" there is an oval, enter "jpg". This should find all of the jpg files in your home directory.
3) - Now, there is a + sign next to a "Save" button, click the + sign
4) - By default it says "Kind" is "Any" - change "Any" to "Images" then you can remove the jpg from the Search oval (do this by clicking in the oval and backspacing, if you click the "X" in the oval you'll wipe-out your search). You may also want to clear your browser caches before you do this as this search will find all images in your home directory.
5) - Open iPhoto and drag the images into iPhoto.
6) - Repeat on the external drive.
There may be an easier way that someone else knows about, but this is the way that I'd do it.