Where is the "Default Wallpaper" wallpaper stored

I don't think this is a default wallpaper, or at least I don't have it.

You can get the location of your current wallpaper by executing gsettings get org.gnome.desktop.background picture-uri as an user who has set this wallpaper.

I've never heard of "dynamic wallpapers" in Ubuntu so I hope this method still works for you.

Indeed - this is part of an automatic picture changer consisting of three JPG's gradually cycling from one JPG to another over certain time-periods:

• 07:00 for one hour fixed at morning.jpg
• 08:00 for 5 hours - gradually transitioning from morning.jpg to bright-day.jpg,
• 13:00 for 5 hours - fixed at bright-day.jpg
• 19:00 for 6 hours - gradually transitioning from bright-day.jpg to good-night.jpg
• 00:00 for 5 hours - fixed at good-night.jpg
• 05:00 for 2 hours - gradually transitioning from good-night.jpg to morning.jpg

This is driven by an XML file located in the Adwaita theme folder /usr/share/themes/Adwaita/backgrounds/adwaita-timed.xml

Have a look - you can use this XML as a template for your own wallpaper changer

The wallpaper always comes accompanied of a theme or in a package, in the case of these is the gnome-themes-standard-data package which has all the wallpapers:

[...]
/usr/share/themes/Adwaita/backgrounds/bright-day.jpg
/usr/share/themes/Adwaita/backgrounds/good-night.jpg
/usr/share/themes/Adwaita/backgrounds/morning.jpg
[...]

The complete list of all files in the packages can be found in the package list. The images correspond to the 3.12 version of Gnome 3 wallpapers.