I'm going to be guilty of dragging up old threads here, but this is a topic I do know something about
In essence, the Copyright, Design and Patents Act (CDPA) states that copyright vests with the original author (and that relates to whoever created the image), unless:
1. It was created in the course of employment, in which case it would normally be owned by the employer; or
2. It has been assigned to another person.
Note that 'in writing' does not have to be a formal letter, it could be simply written into the T&Cs that you sign up to when you join the forum... see the recent uproar when Facebook tried to include a right of authorship over any photos people put up.
The forum terms here mainly cover liability, and don't mention copyright. As such, any image you put up remains your copyright. There is no need to claim it or register it.
'Use for profit' is not relevant, any unauthorised use is a breach of copyright and can be pursued legally. The difference may come in the size of the remedy, i.e. if your images had been used, without permission (even acknowledgement is not permission) then you can sue and could be entitled to a portion of the profit made from the sale of, for example, the book where your images had been used.
Here is the expensive bit, there is no legal aid for copyright, and you need to be able to prove it was your image. That can take time, and solicitors charge in 6 minute time blocks... most of the time a simple 'cease and desist' notice is enough.
Registered trademarks are a different issue. They do need to be registered and protected. This provides the evidence necessary to prove, on the balance of probability, that you are the original owner.