Change the language to English. Then can modify/
What language was your 1Password set to before you switched to English? I would like to test this to make sure this doesn't happen.
The language before I switched to Eng was Simplified Chinese.
In Chinese version, you can't change the avatar through web version.
I have just tried this here and I was able to change my avatar even when I set my language to Chinese. Do you remember the error message that you saw when you tried to do this on your machine?