A US registered trade mark is only valid within the USA and there are no UK registered marks (registered by Image Ten or anyone else) using the words "Night of the Living Dead", so basically you can ignore their demand. That is assuming that you are not using somewhere like eBay.com* for your shop. Sites like eBay apply their own rules on alleged intellectual property infringement, and if Image Ten use that process, your shop may get suspended. However you have a right of appeal if this happens. You would then need to explain the situation to eBay as they will probably just take the registered marks at face value and assume that they apply to you, even though you are based in the UK and no doubt sell most of your products to UK customers.
Even if you are selling to US customers, Image Ten have a very weak case for saying there may be confusion so you can pretty much brush them off as they should realise it's pointless trying to sue you in the USA when you are domiciled in the UK. The only thing they would have in their favour in a confusion claim would be the fact that their registration No. 3,686,876 is for tee shirts, and so is the same type of goods as your product. You can ignore the other two registrations as a mark which is suspended has no validity as far as bringing a cause of action is concerned, and a mark registered after you began using your trading name has to overcome the defence of prior use (see 15 U.S. Code § 1115 (b) (5)) .
As for their comments about the images of actors, they are probably referring to the legislation in some US states (like California) which protects personality rights such as a person's likeness from being used in connection with trade in goods or services. However those statutes have no effect outside the State concerned. And since Image Ten don't seem to be claiming an interest in those personality rights, it's pretty irrelevant and probably just there to try and scare you off.
*eBay.co.uk would not face the same issue, as trading here is governed by UK law. "Contracting entity and responsible authority within the meaning of the data protection laws for eBay members in the UK: eBay (UK) Ltd, 1 More London Place, London, SE1 2AF"
'Is it legal', 'can I do this' type questions and discussions.