I was browsing around here and saw this thread Table: Default spacing to contents? which remembered me a big question I had and couldn’t find out the answer.
Basically I want to do this
as a default option in Libreoffice Writer. Is there any way to do so?
Thanks for your time 
You need to make the changes in a template and then set this template as your default template to get the changes into new document. For each existing documents you have to make the change directly.
To change the settings in the style, set the margin in one image to the desired 0,20cm and then update the style, while the image is still selected.