Are bailiffs allowed to take pictures of my finance car? And what can he do with the pictures?

Nothing

Bailiffs may only take control of goods of the debtor, and hired goods, or anything that is on hire-purchase without the option to purchase being exercised.^[1]

Vehicles on hire-purchase cannot be taken or sold by bailiffs.^[2][3]

The bailiff cannot do anything with the pictures of a hire purchase car and they have no monetary value.

It is possible the bailiff intended to make enquiries whether it is owned by the debtor.

If you see any suspicious activity in the neighbourhood or unexpected loitering, then try and get some pictures of the people loitering. It can be reported to the police on the non-emergency 101 number.

^{[1] Paragraph 10 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[2] Helby v. Matthews [1895] AC 471 (paywall)
[3] See Bailiffs and hire-purchase cars}