Worked it out.
In the admin panel:
then tick the 'upload files' box for registered users.
As stated in the title, i want to paste several images, which are narrow, in one line.
As i can see, Markdown adds to each image class "img-responsive", and that's why every image appears on a new line. As the name of class, if i will remove this class, i will get problems with responsive design.
I think it's a good idea to add a "responsive table" tool in Markdown, so users can make their posts neater. Comments like this looks ridiculous.
Or maybe i already have possibility to paste images in line and just don't know about it?
And the second question: How to prevent users posting thousands images in one post to break the forum ?
Thanks for your time.
as temporary solution added to custom.css: