It must be here somewhere.

How do I put a picture under my name when replying to posts, or making a new post?

It must be here somewhere.

How do I put a picture under my name when replying to posts, or making a new post?

Click "My Controls" (top right of the screen)

At the bottom left of the resulting screen is "Personal Profile" choose "Edit Avatar Settings"

There are some restrictions as to what you can use