Does anyone know why my user picture is blurry on login but not when the screen is locked?

Hello, Henwi_.

Welcome to Apple Support Communities.

From what I'm understanding, your user photo becomes blurry after turning on your Mac to login. I may be able to help and also, your troubleshooting step is a good start to help narrow down the issue. I would check this article out: Change a user’s picture on Mac

Try one of these steps that are listed:

Use a picture included with macOS: Click Defaults, then select a picture.

Take your picture using your Mac: Click Camera; when you’re ready, click Take Photo.

Use a picture from the Photos app: Click Photos. To see specific photos from a time, place, or album, click a disclosure triangle below photos, then select a group of photos. Select a picture, then click Next.

Use a picture from the Photo Booth app: Click Photo Booth to see pictures you’ve taken using your Mac and the Photo Booth app. Select a picture, then click Next.

If these work and you don't experience the issue, then it would be the photo.

I would also see if this happens with a test user account along with resetting the NVRAM:

How to test an issue in another user account on your Mac

Reset NVRAM or PRAM on your Mac

Let me know how it goes.

Cheers!

Hello, Henwi_.

Thanks for getting back to us. The next step you can try is reinstalling macOS. This step won't erase any data and can help narrow down the issue. Prior to any steps, you should always make sure to have your data backed up: Back up your Mac

Then you can follow these steps to reinstall macOS: Reinstall macOS

Try this and keep us updated.

Have a good one!

Hello, Henwi_.

Thanks for going through all those steps on this issue. It might be best to get in touch with Apple Support who can look further into your icons being blurry on login.  

Cheers!

I think that the problem is because you just logged in it might still be loading i don't think you should have much of a big problem