I can't reproduce this issue. Could you provide more specific steps? Could you also check the OS version in chrome://version? Thanks!

Can't reproduce the issue on Google Chrome:71.0.3572.0 Platform:11143.0.0 pantheon

People Were Saying They Couldn't replicate the issue, so here are more specific steps (Plus another picture, more specific)
1.the area on the bottom right of the screen (With the clock and WiFi and volume) and Click it.
2.On the pop-up that appears, There is a lock button next to the power button. click the lock button (See Picture)
3. Log back in and you are free to change the wallpaper without administrative restriction.

Deleted: Drawing.png 143 KB

	Drawing.png 143 KB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Re #8, thanks! I think you may have turned on ephemeral mode via the admin panel. If so, this is the same issue with crbug.com/825237.

Did you observe the default wallpaper at lock screen?

The Default Wallpaper Was Observed At The Lock Screen, As Well as Any Time After The Lock-Screen. Once The Lock-screen is exited, The before administrated wallpaper is changed to the default, and is free to be changed

Also, With This Particular Instance, I Did Not Tamper With Ephemeral Mode.

Thanks for the feedback. This could be a image decoding issue. Do you mind filing a feedback report the next time you see this? Because we'll need to see the attached system logs for further investigation. (After exiting lock screen, press alt+shift+i, and enter some keywords such as "wallpaper", "policy" etc. Please note the shortcut does not work on lock screen.)

It Seems The Problem Was Patched! Thanks!

Thanks for checking! Marking this bug as "Closed", please feel free to re-open if you have any issues with this.