Weird Image compression with includegraphics

I’m using LaTeX for a lot of chemistry stuff so I include lots of reaction mechanisms which are saved in .png.
These files usually have a width of 10,000 pixels.
If I include them with

includegraphics[width=0.75textwidth]{}

they have a very bad quality in the output pdf.
Ist there any explanation for that or even better a fix?

picture out of the molecule draw software, resolution: 12138 x 2118 pixels:

Output in LaTeX pdf on DIN A4

PS: It doesnt matter if I use width=, scale= or Height=. The output is always bad.