In addition to using the File > Open command to display images, you can drag them from the Windows Explorer into the DjVu Solo window.
To open a file by dragging it from Windows Explorer:
Open Windows Explorer and DjVu Solo, and position the application windows so that you can see both of them.
Navigate to the folder that contains the image file you want to open in DjVu Solo, and then drag it from Explorer to the DjVu Solo window.
The image appears in its own window, which you can then encode as a DjVu file.