On snapchat, you can slide to the left and click on the users to send them a picture, but if you first make the picture and choose tho which person you want to send it, a page opens. On this page, there are the users, and there are some users which are under the heading "Best friends" because you often send pictures to them. If there are disbelievers under this heading or girls and someone chooses them from there, is it as if he has said that the sin of taking disbelievers as friends or girls is good, or not? Please answer, because I would never say about a sin that it is good or so.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

As long as the picture that the person sent does not include anything prohibited, he bears no sin for choosing those whom he wishes to send it to. This does not involve taking girls or disbelievers as friends. However, the person should carefully choose the material that he sends on such websites and apps and make sure that they are good and Shariah-approved. It is even better if he sent material that calls to Allah, underlines the manifestations of His omnipotence in the universe, and the like (of good and rewardable purposes).

You should also fight the obsessive doubts which you seem to be suffering from, as getting carried away with them leads to grave evils.

Allah knows best.