I was working on my blog earlier and was creating some images for a new post (so was temporarily working offline for about half an hour). I pasted the new image into the post and clicked preview post to see how it looked. I am currently using Templatic's WP-premuim theme. Instead of getting the post previewed this came up: http://claireharbour.co.uk/wp-content/uploads/2011/12/Screen-shot-2011-12-10-at-17.25.47.png
I don't know If there is an access 'hole' for hackers to do with the theme, but I would like to know how I can find and delete this hacker and prevent further hackers. The screen above no longer comes up, I think that they may already have created an access point for themselves and other hackers, I'm really worried. It would be great if you could help me or pass me over to someone who would know how to deal with this issue.
[Moved to How-To & Troubleshooting]