So, i've had some troubles with TimThumb (it was included in the theme) not displaying any thumbnails on my posts at all. So I recently gave up on it to use what I assume is wordpress own script.
I have Thumbnails now on the posts (main page) and sidebar (most recently, most popular widget) but there is a weird picture above the topic when you click on the post. here you can see it.

The picture above Hello world, where does it come from and how do I remove it? I appriciate any help!