Okay, so, when I choose a theme on my forum, and it says "Black rain" on the top on the main section of the forum.
How do I change that and put another picture there? I don't want black rain up there.
You need to change logo in that theme.
Better ask in theme support thread : Assuming its blackrainV2 .. http://www.simplemachines.org/community/index.php?topic=189576.0
Specifically, you want to replace the file Themes/BlackRain/images/brlogo.gif with a graphic of your own.