It's shaping up to be a dirty fight. The UK election has only just been called, but already the gloves are off. Yet, while the parties are showing their willingness to be sneaky, they're not being particularly clever about the way they use photographs.

First, there was the debacle about the appropriation, first by Labour then by the Tories, of a publicity image from the BBC's series Ashes to Ashes. At the same time that the Digital Economy Bill was going through the wash-up, the main parties displayed a callous disregard for copyright. Personally, I think they should have their Internet connections yanked.

Now it's Northern Ireland's turn.

The DUP ran a poster campaign featuring the image of a young woman and the text, "I want an MP who answers to us - not to the Tories" followed by "I'm voting DUP".

Well, fair enough, you might think. The woman has a perfect right to vote for whomever she wishes.

The more astute will think, "Well, she's obviously a model, but so what?" Quite.

The DUP's rivals, an alliance of the Conservatives and Unionists, thought the same. Not only that, they found the photolibrary from which the DUP had licensed the image. And they grabbed another image, clearly from the same session, and ran their own poster:

Oops. So, how could the DUP have avoided this subversion of its campaign?

Easy. The party could have hired a photographer to do a commissioned shoot. That way, they could have controlled the use of all the images from the session. By paying a decent fee to a professional photographer, they could have obtained exclusive rights to the images - at least for a sufficiently long period.

I don't know why they didn't do this, but I'm prepared to guess. They were too tight-fisted.

So they went to a stock library. And don't get me wrong - stock libraries are fine things. I make part of my living selling images through Alamy. But it's all about using the most appropriate method for acquiring the images you need.

It is possible, even when buying stock imagery, to negotiate exclusive rights to a set of images. Naturally, it costs more than buying non-exclusive rights to a single picture - but look at the trouble that can cause.

It's so easy, now, to buy the use of an image from an online library. And so cheap. But that doesn't necessarily make it sensible. I guess you get what you pay for.