You can change your profile photo* by clicking the green ‘Edit Profile’ button in your profile view. Click on the blue ‘Edit’ button in the ‘My Profile’ tab. Then click the ‘Upload Photo’ button.
Make sure to press the green ‘Update’ button located at the bottom of the page after you have uploaded the new photo.
*It is important to upload a good photo, since this is the first impression other users will get from you & it has a big influence on whether or not a user will request or accept an adoption with you.