8 CFR § 333.1 - Description of required photographs.
(b) The applicant, except in the case of a child or other person physically incapable of signing his or her name, shall sign each copy of the photograph on the front of the photograph with his or her full true name, in such manner as not to obscure the features. An applicant unable to write may make the signature by a mark. An applicant for naturalization must sign the photographs in the English language, unless the applicant is exempt from the English language requirement of part 312 of this chapter and is unable to sign in English, in which case the photographs may be signed in any language.
(1) If a child is unable to sign his or her name, the photographs must be signed by a parent or guardian, the signature reading “(name of child) by (name of parent or guardian).”
(d) The photographs must be signed when submitted with an application if the instructions accompanying the application so require. If signature is not required by the instructions, the photographs are to be submitted without being signed and shall be signed at such later time during the processing of the application as may be appropriate.