I heard multiple times, that if a picture is unnecessary in a question, then remove it, well even if it's at the bottom, small and doesn't distract from the main parts, as it's at the very bottom of the question. I'm kinda confused about this, can you clarify me why all unnecessary, but not distracting pictures need to be purged with hellfire?
No hellfire, just basic maintenance.
We don't like stuff that degrades the signal-to-noise level. That's not just true on Worldbuilding; it's true across the network. Stack Exchange sites are really good at focused Q&A. This is why even things like salutations and signatures get edited out of posts; they're not necessary, to say nothing of long rambling tangents that aren't really about the question or answer.
Pictures are particularly egregious because they're large and eye-catching, so they distract from the text. If a picture helps clarify or illustrate the text, then great -- we want diagrams, pictures that show that important detail that lets you understand an answer, and so on. But we're not looking for wallpaper and gratuitous game/movie/anime stills.
Here is an example of a question where I edited in the picture, because at least for me it wasn't completely clear what the question was asking until I saw the picture:
As you can see, it's pretty minimal, just enough to get the point across and nothing more.
If you can get your point across using just words then that's preferable. If it's a complicated enough concept then a little illustration can be a help.