for high res rally images Alan i Charge £18, that's 1600px MAX, 500px = £5.00
I charge alot for a high res as it almost exclusively rules out the chance of selling a print of that shot to that person, as you can ILLEGALLY print from a 1600px image, however how the hell are you going to find out if they print it for their bedroom wall.
Keep a rough eye on them and their activities, I sold one to a driver sponsored by a Tool Distributor, so i whacked the price up as he asked already about using the image in the catalogue of his sponsors. That image was £30, with Strict terms saying the image will not be used for anything but personal use, no re-production (the standard bumpf i send with a digital image) and then included a rather hostile part about using the image for commercial reasons would 'in-cur a charge of £750 for breach of copyright' blah blah blah
I'm very carefull selling digital images, it's just to easy to take the ****
