Document image sizes required for all sections including responsive, set standard image naming policy

[Tomasz-Kluczkowski]

At this moment in time the image sizes and names are not set in stone. I would like to have it written down so that layouts are not mismatched and that anyone creating a pull request with their own material to be added matches the design of all the other already existing subsections of the same subject.
Image names at the moment are constructed as this:
<image_name>_xpx.
example: logo_99x68px.png
I believe that we should add some kind of info to know for what orientation image is dedicated (landscape / portrait / both) and put that into each file.
This is related to: #90