A few points of law... (1)

Firstly, is it illegal to have had possesion of an extreme image (opposed to currently having an extreme image). i.e if a prosecution could prove that you downloaded a image, is that act in itself illegal even though you may no longer possess it?

Secondly, where does the act stand with regard to browsing extreme images on a website. I suppose a prosectution could argue that the images were downloaded (albeit temporarily) in the cache. But does this constitute 'possession'. This goes back to my first point - if the image is downloaded and then removed, can a prosecution be brought against someone who once possessed an image.

Thirdly, where does the act stand on streaming media. This is also cached, as in my second point, but not enough information is downloaded for the video to be rebuilt from the harddrive. So a defence could argue that the stored information is meaningless.

AGHHH! Confusing!

Having an image on your hard drive constitutes possession, I believe. One point noted by The Register is that having deleted the image isn't necessarily a defence, if you can still access it. So if the police find undeletion software or similar forensic/recovery tools on your PC - or if you work in the IT trade and therefore have the technical knowledge to do so - then if they can recover illegal images, they may still charge you with possession on the grounds that you would have been able to recover it at will too.

However, it would be very hard for them to prove that you had downloaded an image that they couldn't then recover later - in most cases, they can't prove who was sat at the computer when it was downloaded. This has been successfully used as a defence in child pornography cases - particularly, the case where a Soham policeman was found to have child abuse images on his work laptop.