two users with one avatar?

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and change the pictures for both users from there. Reboot normally and check to see if the changes stuck.

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.

Hello marjolijn241,

It sounds like you need some help changing your login picture on your Mac.

We will be happy to provide some guidance.

We recommend that you try the steps in the link below.

Change a user’s login picture on Mac

1) On your Mac, choose Apple menu  > System Settings, then click Users & Groups in the sidebar. (You may need to scroll down.)

2) Click the user picture on the right, then do any of the following:

*Select a Memoji: Click Memoji, then click the Add button  to select and compose a facial image. Or select one of the displayed Memoji, then select a pose and style you like.

*Select an emoji: Click Emoji, then click the Add button  to select an image from the emoji library. Or select one of the displayed emoji and select a style.

*Select a monogram: Click Monogram, select a background color, then enter initials.

*Take a picture using your Mac camera: Click Camera. Set up your shot, then click the Camera button. You can retake the photo as many times as you need to. See Update your user picture using Photo Booth.

*Select a photo from your Photos library: Click Photos. To see photos from a specific album, click the arrow  next to Photos in the sidebar, click the album you want, then select a photo.

Tip: To quickly replace the current Apple ID picture with an image on your Mac, just drag the image from the Finder onto the current picture.

*Select a suggested image: Click Suggestions, then select a picture.

3) After you select an image, you can adjust its look. Do any of the following:

*Adjust the image location: Drag the picture around within the circle.

*Zoom in or out: Drag the slider left or right.

4) Click Save.

If you’re still experiencing the same issue, try setting up a test user to see if you're still experiencing the same problem.

Set up users, guests, and groups on Mac

If the original problem persists after trying the previous steps, please provide us with specifics on what you see for more help, and we will continue from there. 

Thanks for using the Apple Support Communities.

Cheers!

How to use safe mode on your Mac - Apple Support