When I choose a new, it uploading, shows up in the webpage (timthumb) but not showing in the page when I open the page (bigger picture on top of page)?
I still don't fully understand. Are you saying that is shows on the blog frontpage
, but not in single post view
First of all, read this explanation about default/featured post images. There it states that images can be added in 5 different ways, and for it to show up as featured (span over the sidebar) the width must be at least the width of the page container, and that it shows up in the content area if it's narrower than the page container, but as wide as the content container. In other words, if the image is not as wide as the content container it will not show. Instead it can be inserted into the post editor, and thumb images can be added to the "Thumbnail image URL" field below the editor.
If timthumb is not working as it should it will not show up either, see the troubleshooting at the bottom of this article to fix this.