Q:

Where does wallch save the list of pictures and folders it takes pictures from?

I want to write a bash script which can remove the current wallpaper from the wallch playlist. By playlist, I speak of that list in wallch to which you can add pictures and folders, telling wallch that it can use these as wallpaper.
How is this playlist stored by wallch? sqlite? xml?
Can I access and modify this playlist programmatically?

A:

It would be rather difficult to remove the current wallpaper from Wallch's list while Wallch is running and chaning wallpapers, as the list is accessed by memory (also saved in ~/.config/Wallch/MainWindow.conf, but it is not read by there live, this location is used to restore the playlist, if e.g. computer is crashed or program is not shut down properly).
It would be much easier to make a script that autochanges the background while reading an album.wallch file using the command 
gsettings set org.gnome.desktop.background picture-uri "/path to image/here.jpg"

and a 'while' loop. Example:
while read line; do
   gsettings set org.gnome.desktop.background picture-uri "$line"
   #here do what you want, remove image from the file being read, move image to trash
   #(located in ~/.local/share/Trash/files) and mark it as not wallpaper material.
   sleep $timeout #<- sleep the timeout you want until the next image
done < album.wallch

Another solution, but too complicated would be the following:

Start Wallch normally and add some images, if there aren't any in the list
Quit Wallch (Ctrl+Q)
Start Wallch in a terminal using the command wallch --constant
Remove the corresponding wallpaper from ~/.config/Wallch/MainWindow.conf
Do whatever you want to do.
Repeat from step 3.

But I would really suggest the 1st solution, making your very own script for this, if you want to use Wallch only for changing the background image, you should totally use the direct command
gsettings set org.gnome.desktop.background picture-uri "/path to image/here.jpg"

