Under your profile, you have edit photo or upload photo button. when you click it you will get this "Upload Profile Photo
* jpeg, gif, or png formats are allowed
* Original image can be no larger than 8 megabytes
* Image dimensions should be a minimum of 200x200 pixels
* You will get a chance to crop your image after you upload it
* Adult content is not allowed in profile photos
If you photo is big then edit them using graphic software to fit the recommend size.