There are two ways to select images for watermarking

Add images

Click on the Add Images button or choose Add images option from the File menu. It will present you with a file browse window to choose images to add. You can select multiple images in the file browse window.

Add folder

You can add all images from a folder. Click on the Add folder button or select the Add folder option from the File menu and you will be presented with the following window

You can select a folder from which you want to add images. You can specify if images of certain type only are to be added and whether to add images from sub folders.

After the images have been added the selected image shall be previewed on the main window from where watermarks can be added.