Does anyone know what are these zip files? Is it safe to delete them?
Those are thumbnails dispayed in Sitecore backend.
Do not delete them.
First time Sitecore needs any thumbnail, it extractss one of those files and gets the proper thumbnail with requested size.
E.g. when you open Content Editor and select
you can see that icon it uses is
You can extract
Network.zip from that folder you mentioned and in
16x16 folder (size of the file) you will find
home.png icon which is displayed in Content Editor: