Default Desktop Pictures usually changed for every new version of Mac OS X or macOS 10.
If you have a mac available to you that is running an older version of 10 and has the Desktop picture
required open the Library/ Desktop Pictures folder in the older mac and select the picture you want to use.
Drag and drop it onto a USB flash drive.
Insert the USB in the new mac, create a folder appropriately named and drop the file into that folder.
It used to be easy to just drop the file into the Library/Desktop Pictures folder, restart and then
begin using the added picture. Now it is not quite so simple as the new location is System/Library/Desktop Pictures
anything in that section is set as Read Only and cannot be added, edited or deleted.
To add it as a Desktop Picture open System Preferences> Desktop & Screen Saver> Desktop
click on the + button and from the drop down menu you can navigate to the folder you created
with the picture you want to use. The Desktop picture will now change to the one you want.
If you do not have access to a mac running an older OS with the Desktop Picture you want
search the internet for old mac desktop pictures, download the image, make sure it
is a suitable resolution and size for your display. Again create a folder for it and select it in
System Preferences> Desktop & Screen Saver> Display.
Try this link, https://512pixels.net/projects/default-mac-wallpapers-in-5k/