its a grey area all round.
The best way is to Ask first.
I wouldn't mind people using images if they asked first but when big companies with a budget steal your work it pisses me off,
On a related note... do I remember you mentioning a standard invoice you send in such circumstances? Did you pull it from a template or put something together yourself? I'd be curious to see the format and the fees!
I’d be interested to see a sample invoice too Pete, as I have to send one to the British Museum for the use of one of my photos (they did ask nicely and offered a fee).