PNGs inserted into LibreOffice documents (of any kind) look terrible, even unreadable in many cases. If the document is exported to another format, e.g. PDF, the images look great.
I’ve never encountered software that does such a poor job of rendering images – it looks worse than amateurish. There was a time when this problem didn’t exist, but it changed several years ago. The bug is listed here:
It’s acknowledged to be a serious issue in the report. I can’t convince anyone to work with LibreOffice once they’ve seen this issue. I’m on the verge of giving up myself – how can such a terrible bug be left open for so long?