If you'd like to change your profile picture, simply go to your profile page by clicking on your username in the top right corner. From there, hover the mouse over your picture until you see the word 'Change'. Click that, and you'll be given the options to select an image from the gallery or to upload your own picture. :)
Once you're done with whichever option you choose, click 'Save Changes'. Please also make sure that any pictures you upload are appropriate and fit within the Community Guidelines.