Browsing around social networks and this community, I see that there are users that use pictures of actors from TV series as their avatar.
I don't want to point to anyone's profile to give an example, but I saw any kind of copyrighted material used as icon or avatar: characters from animated cartoons or videogames, actors faces taken from frames of famous TV series, softwares icons, famous paintings (full or details), etc...
Is it legal to use copyrighted materials for a "small thing" as an avatar or icon?