The button was temporarily removed as the option to upload your own avatars was not permitted on this forum anyway, for security reasons (so the button was misleading and promising something that did not work).
On the help page - https://forums.opera.com/help - the very first item explains that Gravatar is the source of user pictures here. Which is what majority of people use, I think.
The ability to upload your own avatars was re-enabled.
Post in the correct forum.
Be clear and concise when describing the problem. Be specific and remember to include important details, such as the exact error message or address of the site you're having trouble with.