You should concentrate on the following points:-
1. Display quality - try to see a few machines working side by side.
2. RAM - you'll need as much as you can get for working with images.
3. Hard drive size - the bigger the better, but most machines will come with a fair-sized drive anyway.
If you can run to about £1100/1200 go to your nearest PC World and ask them for an Advent 7039. Then go home sure in the knowledge that you have bought a great machine in terms of value for money. Don't buy any extended warranties - you'll be adequately protected by consumer legislation should anything go wrong.