Select the Images on your laptop. "File➞Export as Library". Move this newly-created Library to your iMac. Open Aperture. "File➞Import Library".
(Added:) Full instructions on this page from the User Manual, under the heading:
To transfer projects and albums from one computer to another
Message was edited by: Kirby Krieger
Do you mean export 20 images as a library to the external drive, eject/re-connect external drive to iMac + import...what...images...library... from drive? to where? as what?
I am not clear about how to export/import, what I end up with (images? a library?) and how do the 20 images find their rightful place?
When you select the 20 images and export them as a new library those images along with their original files and any metadata you added will be made into a new Aperture library. This library will be a self-contained library capable of being opened in Aperture.
When you copy that library over to the iMac you will open the library you want to add the images to and then in that library select File->Import Library and then select the library you copied over. All the information from that library will be added to your iMac library.
Once you have imported the library and you are sure it is OK you can delete it from both the iMac and the laptop.