Plugin: Polylang

I use the polylang plugin for having a poly-langual website.

Now I want to customize the flags. So i uploaded some png-files with my picture in to the wp-content/plugins/polylang/flags with the same name and the same size there were in this folder. But there wasnt a change. I read a post, that I have to put it in the file wp-content/plugins/polylang instead of “flags”.
I cleaned the cache.
But nothing happened.

Do you have an idea, what the solve for my problem is? Thanks a lot.