You can use checkboxes in the list to activate or deactivate individual images.
If they're checked, they'll be considered active when wallpaper is chosen for the desktop.

Buttons for management of Wallpaper Sequencer (on the right side of the wallpaper image list)

Add picture	Lets you browse to add one picture (of a supported format) to the list.
Add folder	Lets you browse to add all pictures in supported formats to the list from selected folders and subfolders.
Delete selected	Delete selected pictures from list. (The actual images are not deleted, just their entries in the list.) or delete selected picture to the Windows Recycle Bin.
Load list	Load list from a file that you already created and saved.
Save list	Save list to a file. You pick the name and location.
Append list	Load the list from a file and add it onto the end of the current list.
Clear list	Delete all current list entries from memory. It's a quick and easy way to start fresh and create a new list. Clearing the list will not affect lists that are already saved to a file.
Previous wallpaper	Set the previous wallpaper image in the list as the current wallpaper. However, if you're set to a random sequence on the Walser General Options tab, it will just pick a random wallpaper.
Set wallpaper	set to desktop selected element in wallpaper list.
Next wallpaper	Set the next wallpaper image in the list as the current wallpaper. However, if you're set to a random sequence on the Walser General Options tab, it will just pick a random wallpaper.
Clear desktop	Remove wallpaper completely from the desktop.

Four buttons for managing the list:

From left to right: a) Move selected image(s) to the top of the list; b) Move selected image(s) one line up each time you click the button; c) Move selected image(s) one line down each time you click the button; d) Move selected image(s) to the bottom of the list. Click the Sort button to sort the list by image type (BMP, JPG, etc.)

Click the triangle on the "Image options" panel and you will see a list of individual options to position each wallpaper. These options override settings on the General Options page and apply only to the image you have selected in the list. "Default" leaves the current image set the way you have it set on the General Options page for all images.

If you check "Preview", you can see a Preview window on the right or left side of the main window. Unchecking "Preview" hides it. You can change its position by using the button. The arrow points to the side that the preview window will be on if you click it.