About 20 percent profile pictures present people's idols. We can say quite a lot people are using their idols as their profile pictures. It doesn't seem like people can give others a good impressions about themselves by using idol pictures. Then, why do they choose those pictures? From my point of view, this kind of pictures are used to help people find others who share the same idol. The purpose of using idols as profile pictures is kind of like the purpose of using hobby-related pictures. People can easily tell others what celebirties they like and find others who share those similarities.  Those profile pictures can help people make friends with people who have common topics.

Conclusion

I think the biggest purpose that people choose specific pictures as their profile pictures is that they want to leave other people a good impression, no matter if the impression is true about themselves or not. People might want others think they are good-looking; they might want to show that they have very broad interests; they might show their shining personalities, etc. However, besides those points, we can see profile pictures are also largely used to help people make friends. People want to attract more strangers and make friends with people who they like by showing strangers their profile pictures. In addition, some people also use profile pictures to commemorate their families or friendship.