If you want to add/replace images in slideshow of type "Get images from directory" you need do the following steps:

1. Upload new images to directory where these are stored in fileadmin. In this case this is /fileadmin/user_upload/topimages/:

2. Select List mode in main menu. Click in pagetree sysfolder where slideshow is stored (folder Frontpage slideshow on screenshot below) and open Flash SlideShow record for editing:

3. In Select pictures field you will see 2 boxes: Items and Selected. Click on newly uploaded pictures in Items box to add these to Selected. Click Save button when changes done and Clear cache.