If you look at the main menu items across the top of the forums screen you will see a link to "My Stuff" and a small arrow pointing downwards.
Click on the arrow to get a list of options and select "Edit profile".
Scroll down to the bottom of the page and click on the appropriate radio button.
Either paste in the URL of the image you want to use
or choose a stock image.
Click on "Submit"
This doesn't always work for me in Chrome so if at first you don't succeed, try Firefox.

I hope this helps.
PS. Please don't use bold or coloured fonts for the whole of your messages in the Forums - bold should only be used for emphasis, and colours make the text harder to read for some users.