You should be able to just click on your avatar while viewing your profile and upload a photo. It will save without any further action. Or at least it should...
OK, got it to work but not sure how. I thought maybe the size of the image was too great, so first increased compression (no better) and then decreased size of the original without extra compression. Still the avatar in the top right had not changed, so I clicked that and the image was saved. I don't know which of these actions made the difference. Previously I had tried clicking on the top right avatar without effect.