I can see the picture field in the user table, and it seems the field accepts a picture address. I want to allow users to upload their picture to the site.
Is there any module or code to use this field?
Check the user account settings in page admin/user/settings
, the option for changing user picture is "Picture support."
User pictures, the pictures associated with a user account, are handled from the User module.
You need to enable their support with the "Picture support" option you find in admin/user/settings.
The support works if the directory for the files has been correctly set, and Drupal has the permission to write files in that directory; the path reported in "Picture image path" must exists.