How do I move pictures from one library to another?

For just a few export them out of Library A via the File ➙ Export ➙ File Export menu option with Kind = Original and import them into Library B.

However, this will not include any edits or metadata (keywords, titles, etc.) that you've added.

To get the original image, any edits and all metadata, keywords, titles, descriptions, places, etc., you'll need the paid version of iPhoto Library Manager. It can copy albums or events from one library to another and keep those items I mentioned earlier with the photos.

OT