But it seems I cannot add the image through the web-interface?

Episode Backdrop
Click on the Vampirina link, the link you posted here.
https://www.themoviedb.org/tv/74323-vampirina/season/1?language=de-DE
Below each Episode is the Expand shortcut.
Click on Expand from episode 1 and it will be expanded with more options.
Click on the Images option.
You will be sent to this page.
https://www.themoviedb.org/tv/74323-vampirina/season/1/episode/1/images/backdrops?language=de-DE
On the left side of the Backdrops, inside the dark box, you can see a small plus sign, +, inside a circle.
Click on this + sign, read the instructions and good work for you.

EDIT

Season Poster
You posted the Season 1 link.
Here is the link for Season 2.
https://www.themoviedb.org/tv/74323-vampirina/season/2?language=de-DE
At the top of the page is the Media shortcut.
Click Media > Poster
https://www.themoviedb.org/tv/74323-vampirina/season/2/images/posters?language=de-DE
You will find the same + sign.
The rest already explained above.

Okay, _that _was misunderstanding. Been there, but it only allows to upload a (large) "background image", whereas Disney and others only have smaller ones. And I didn't mean to upload a background image but a season image, so I stopped and got lost there. On DisneyNow, the images come 620x348, which is below minimum size. But in the images information it says that smaller is better than none - to which I agree. But those have been Season 2 images. And on that season 2 there needs to be another correction before images (or German text) can be uploaded.

On DisneyNow, the images come 620x348, which is below minimum size.

FYI the minimal size for episodic images (stills or screen captures from the episode) is 400x225px.