Hi Sujeet,
please excuse the late response. There is now another post online – same problem.
The original image was re-uploaded several times and was always the same size.
But what is interesting now: as soon as the image is selected as a post image, it is apparently changed (by right-clicking and downloading you can still see the addition “640×480” after the name).
But this is probably caused by a plugin rather than the theme, isn’t it?
Do you have any advice for me?
Best regards