Product image displayed as thumbnail?

Hi,

The issue comes from the "Bild" setting there.
In English, the name of that setting is "Image resize during upload (width x height)".
Since you've set it to 200*200, the main image was resized to fit in 200*200 during the upload of the image.
Then, when the system is generating the thumbnails, it take that image to resize it to the desired size.
And since you're asking the system to generate a thumbnail of 600*600 out of a 200*200 image, the system gives you a 200*200 image as it cannot upscale it.
So you should either leave that "Bild" setting empty or enter 600*600 in it, AND, you'll have to delete the image from the product and reupload it after you change that setting so that the image doesn't get resized during the upload.