It depends what "this" was....
Taking a photo of your own postal ballot is OK but not of someone else's - at least I think that's what all this means:
(3A) No person may—
(a)except for some purpose authorised by law, obtain or attempt to obtain information, or communicate at any time to any other person any information, as to the number or other unique identifying mark on the back of a ballot paper sent to a person for voting by post at a relevant election;
(b)except for some purpose authorised by law, obtain or attempt to obtain information, or communicate at any time to any other person any information, as to the official mark on a ballot paper sent to a person for voting by post at a relevant election;
(c)obtain or attempt to obtain information, in the circumstances mentioned in subsection (3B), as to the candidate for whom a person voting by post at a relevant election (“V”) is about to vote or has voted;
(d)communicate at any time to any other person information obtained in contravention of paragraph (c).
(3B) The circumstances referred to in subsection (3A)(c) are where V is about to mark, is in the process of marking, or has just marked, a ballot paper sent to V for voting by post at the election.
[Section 66, Representation of the People Act 1983 as amended by the Election Act 2022)