Actually you're right, That image should be in the Forum Images tab as there is no way to assign the images in the News Images to a forum.
I can not see a way to use those images in that News Images tab on a Forum. Can you add that image to your Forum Images and then try using it that way?
Well, after an hour of futzing around, I figured out how it works. First - you have to have a forum added as Forums to pull news from:
in the Portal Settings. THEN - when you add a new topic (or edit existing topic) to that forum, you will now have an option in the editor to "Add a News Icon". That lets you select from those News Icons. However, it doesn't put it in front of the forum like a forum icon. What it does it puts the icon at the beginning of the article that the PORTAL pulls from that forum. (See Screen Shot that I "think" I attached to this reply.)