Do you mean at what point must you register with HMRC as self employed? As soon as you start selling any images. You should be declaring the income as self employed income, via a self assessment form every year, and pay tax on your earnings.

Read all about it;

http://www.hmrc.gov.uk/sa/register.htm

Thanks chaps - I'm already registered for self assessment there seems to be also an option to become a sole trader - where you can (try) and offset expenses are there any advantages to this or could I do this on the self assessment form?

I guess you can.
I get an accountant to do it all for me. More than worth the money - he more than earned his fees so far by discovering I'd been paying too much tax in my ex day job, which I'd never have known or suspected, and backclaiming it. I just give him a list of my income and outgoings and he does the rest. The form seems to change every year. Last year I noticed that the new categories for expenses had changed and were IMO weird, and I'd never have guessed where to put each expense.

Thanks for your advice with my new found free time lets hope he has plenty of income statements to ridfle through