The Unstandard shows banner, but no pic (timthumb)

Yes, like the topic is saying. When I upload a photo, I want a thumbnail of that photo in the banner of the post. This is not working. The Unstandard theme came with timthumb preinstalled, so I thought that it should already be configured to the correct settings. But it’s posting the “timthumb_blank.png” as if I didn’t upload anything to the post.

The website is currently looking like this: https://graffitiforever.tk

Any help appreciated.

(also I would like to remove the categories at the bottom of the page, but this is no widget so idk what to do)

EDIT: Maybe this post should have been posted in the plugins-direcotry. Sorry.

EDIT2: I now manually edited the picture (in photoshop) in the cache, and overwrote it with my FTP client. But this, this I do not wish to do after every post…