You can set your profile picture in User settings - Account info:

Upload  your profile photo by clicking "Upload photo" sign to help your colleagues easily recognize you or just choose one from those cool pictures to the right. You can choose from one of the collections we have: avatars, funny people or animals. Scroll down to see more options. 

Your uploaded picture must follow the requirements:

Allowed file extensions: png, jpeg, jpg.
Size: up to 2Mb.

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