I have recently developed my website; http://jake-thompson.co.uk but want to have an image that pop's up in the middle of the page that will then automatically close itself in 5 seconds.

Hi,
This is not done in HTML. It is done in JavaScript with window.open(). The bad news is that almost any of the current or recent pop-up blockers will block its display. There is a program that is supposed to get around it with what is call a Pop-over.
Peace,
Clarke

Pop-ups are a thing of the past, use a modal window.

JQuery

Hi,
Try this link:
Modal Image Box
Also see:
http://javascript.about.com/library/blmodal.htm
Though it seems that they require interaction and do not timeout. Know I have seen it done in the past, but cannot remember where.
Peace,
Clarke