For some reason my diablo 3 account is setup for my D3 avatar but when I actually go to post my wow character avatar shows up. I tried the drop down menu but nothing comes up.
Did you use the drop down up top or the one in the reply to thread box? Only the one in the reply to thread box works for changing who you post on.
For D3 as mentioned it is now over to the left in the reply box as a little drop down arrow next to your name. If it is not, then you will want to post over on the Website Bug Report forum where they keep track of issues with the web site.