Whenever you create a league, each team get's team colors and logos assigned to it. 
First OOTP will try to create a team logo depending on the nick name of the team. More about this can be found in the readme.txt file which is in the /logos folder.

If OOTP does not find a specific logo based on the team name, it will use a random file from this folder /logo_templates. These logos can be colored using the randomly choosen team colors.