@palmgrenwirth: when you start a new article/post, did you checked the “featured image” when you’ve uploaded a new image?
Check the overview of all your articles, got “thumbnail” column an image? If not, so you just didn’t add a featured image yet to the article 😛
the “timthumb”-feature is within the theme just need permission (what was my fault :P)
Hope it’s helpfully 🙂