Same Border, Opposite Answers
An ETA is not a visa, and the photo rule inverts.
If you need a UK visa, you supply no photo at all. You apply online, then attend a visa application centre where fingerprints and a facial image are captured on the spot.
If you are visa-exempt and need an ETA instead, the opposite applies: you provide the image yourself, online or through the UK ETA app. Same country, same border, and which one you are in depends only on your nationality.
Singapore, Brazil and Argentina are ETA nationalities. Pakistan, Iraq, Vietnam and Ukraine are visa nationalities. Getting this wrong means either preparing a photo nobody will look at, or turning up without one.