Best way to increase saturation of images printed from LibreOffice.
IMHO, LO is not the ideal tool for picture post-processing. Do that before importing/inserting them in your document.
“Best way” is subjective and a matter of personal preference. Consequently I can’t give you THE solution.
Have a look at specialised applications. The most common tools are GIMP in FOSS world and Photoshop in commercial word, but there are many others.
Some drivers for high-end printers/copiers offer settings for adjusting saturation, gamma and other color parameters (I have seen that for Miko [Konica-Minolta] and a few Canon). It depends on OS and printer.