It does not necessarily mean your UK ETA will be refused.
However, the face photo submitted for a UK ETA application is used for identity verification, so even if you use a selfie, your face must be clearly visible from the front.
The photo should be taken in good lighting, with your face uncovered and a simple background.
Shadows, filters, blur, dark lighting, excessive editing, or a busy background may cause problems during the application process.
To reduce the risk of delays or application errors, it is safer to prepare a clear face photo that is close to passport photo standards rather than using a casual selfie.
You can apply for a UK ETA through ETAPORTAL, so it is recommended to prepare your face photo and passport information in advance and enter all details accurately.