Your existing screenshots are still there; they are not lost. After you install Splashtop Whiteboard v220.127.116.11 or higher, the Splashtop Whiteboard shortcut is still there on your Desktop; and it still opens the Splashtop Whiteboard folder; but now it contains three empty sub-folders: one for Background, one for Flipchart, and one for Snapshot.
The screenshots you captured before, when using a previous version of Splashtop Whiteboard, are also still contained in the Splashtop Whiteboard folder. To make them recognized and available for use with Splashtop Whiteboard 18.104.22.168 and higher, simply move them into the new Snapshot sub-folder. You can do this by Cutting/Copying the existing files and then Pasting them into the Snapshot sub-folder, or you can drag-and-drop them into the Snapshot sub-folder. Any screenshots you save in the future using Splashtop Whiteboard 22.214.171.124 or higher will automatically be saved in the Snapshot sub-folder.