If you hold down your middle mouse button, you can rotate the camera freely around your toon without changing their facing. You can use this to position the camera where you like for screenshots.
In your Options -> Keymappings menu, you can bind keys to "Screenshot (JPEG)" and "Screenshot (TGA)". Pressing these keys allows you to take JPEG or TGA screenshots, respectively.
Bind a key to "Disable UI" if you want to remove the user interface (your trays, nav compass, target window, chat, etc.) before you take the screenshot.
Screenshots will be saved in the City of Heroes folder, or the CoHBeta folder if you're playing on Beta server.