Security of your hard won images is an ever-present concern to the working photographer. There are two lines of defence - prevent its theft in the first place or take recourse to the law after the event. The former is more preferable - going to court (even the small claims court) is a fairly costly exercise in terms of cash, time and stress.
As the originator of an image you are the copyright owner. This belongs to you from the moment you created the shot and lasts for your lifetime plus 70 years after your death. Copyright ownership can be transferred and forms part of your estate on death. It can also be reassigned as a bequest in your will. You can only give up copyright by means of a written contract - your client has no right to assume anything about copyright ownership just because they have paid for the film and your time. The only common exception to this rule is if you are the full time, paid employee of a company and taking photographs is part of your contract of employment - copyright then belongs to the person who employs you.
You are at liberty to grant a client the use of your images under any terms you deem acceptable. If, for example, they have engaged you to take a shot for use across two column inches they are not allowed to then use it in a 42 sheet poster advertising campaign on all the billboards in the UK! A sound piece of advice is to start the writing of your terms and conditions with a complete ban on the use of your image and then work in how it might be used as exceptions.
As the creator of an image you also have Moral Rights. These include the right to accreditation as the creator of the image and the right not to have your image abused or used in an offensive manner.
The problem with all these rights is asserting them. Do you really want to start off a potential relationship with a new client with a 20 page Terms and Conditions document? Do you really want to phone up your well-cultivated client and yell at them because they have used a thumbnail on their web page? These decisions are part of being self-employed and only you can decide just how aggrieved you are at a breach of your rights and how far you want to push asserting them. The best solution to some of these problems is to nip them in the bud at the earliest possible moment. Ask the client what purpose the image is being commissioned for - if they are insisting on a 5x4 transparency for use as a web page image politely enquire if that is all that the image is to be used for. Explain to your client why you are enquiring - you may even have to do some delicate educating as to copyright law. This is less likely with an advertising agency, who should know the ropes (and could be trying it on for that reason!) but near certain with your local Bloggs Rusty Engineering (Dudley) Ltd. Mr Bloggs is most likely acting out of ignorance and commonly thinks that paying for the film entitles him to sole and unrestricted use of what is on it.