Check out the pictures of ticks on this website
Tick images on other websites
University of Lincoln tick identification key - Images and important points to help you tell a larva from a nymph from an adult.
UK Tick Recording Scheme - As well as images and descriptions of British ticks, this site tells you how to collect and send your own samples for identification.
Dartmoor tick watch - Images of ticks collected on Dartmoor. This site also includes high resolution images that show the detail of tick mouthparts.
Tick borne encephalitis web site - Several images, some high resolution, some computer simulations and some good comparison of different stages to show differences in sizes.