Different sizes in post image

Hello, I recently encountered an issue where the post images have different sizes after publication. Example: https://i.postimg.cc/BvJptPKr/example-lif.png (I quickly censored the image that was too large, as it is not yet permitted to be published). Can I correct this display? Is it due to the image or the theme? https://www.lost-in-focus.de/beitraege/