When aPhoto boot up then first thing you should do is import your photos.
Prepare photo path:
Make a directory somewhere you like and put the photos you want to sign watermark into this folder.
Click "import" item on "file" menu pop-up list or "import" button on tool bar. aPhoto will
open a file chooser dialog. Now you browse to the folder which made a moment ago.
Click "Open", all photos in the folder will be imported and you will see your photos on "Photo List" panel.