Unwanted Sequoia wallpaper on iMac M1

Boot into Safe Mode according to How to use safe mode on your Mac and set your wallpaper. Reboot normally and see if it took.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Put a copy of the image you want as your Desktop wallpaper into the Users/Home/Library/Wallpaper folder (if there's no such folder create one). Then in the System/Wallpaper settings pane click on "Add Photos" button and then on "Choose" in the menu.

Navigate to the Users/Home/Library/Wallpaper folder and select your chosen wallpaper.

Hmm, that's not quite my experience with both macOS Sonoma & Sequoia.

In my case, I select a folder (album) from the Photos app. I configure it to shuffle every 15 mins (actually it doesn't matter what time period I use,) and shuffle them randomly.

I then select this as the default wallpapers to use. I works initially, but then after a restart, it reverts back to the Sequoia forest scene ... even though it still shows my selected folder as the default.

The only temporary workaround, was to select each photo in the album so it displays as a wallpaper one-at-a-time, and then choose to rotate through them again ... and even this, does not permanently resolve the issue.

In the image below you can see that the "Seasons - Autumn" from my Photos Library has been set as the default, yet the Sequoia forest scene is shown on my Desktop. Go figure!

FWIW, I have never had my wallpaper arbitrarily change since the first beta install of Sequoia back in June. Something is causing your issue. First update to Sequoia 15.2.

FWIW, all my wallpapers are in various folders in the Pictures folder without any issue.