how to remove the white border from image ??

In the Custom CSS field, add this;

.entry-content a img {
 background: transparent;
 border: none;
 box-shadow: none;
}

In future, for CSS-specific queries consider CSS-specific forums.

Andrew, thank you very much !!! It works !

I have two more questions:

see this website: https://www.luukpotman.nl

(1) How can I remove the dark grey (and under white) frame-line around the image-icons ??

(2) and how can I remove the black line under and above my website ?

thank you for the answer

Luuk

https://csscreator.com/forum

I’m sorry but it is beyond the scope of these forums to deal with general CSS issues. Try a dedicated CSS resource such as https://www.css-discuss.org/ instead. In the meantime, using Firefox with the Firebug add-on for this kind of CSS work.
https://getfirebug.com/