Cambridge in Colour has some handy but quite technical tutorials. Especially those few that delve deep in to the fundamentals of colourspace and the like.
Youtube tutorials vary an awful lot, and can be a bit marmite, and often are more about what you can do in lightroom etc. to turn a photo into something artistic rather than realistic.
What does “improve quality of pictures” mean? Better eye for artistic composure? Different camera settings? Better tweaking in lightroom/photoshop to get more punchy colours?
eg: I quite enjoyed the Gavin Hoey 15min photo challenges on Youtube (if you can handle the Adorama ads in later ones), but really found that French guy (Serge someone), ruined photos far beyond acceptable. Both imho are more photoshop tutorials than photography, even though Gavin gets out and about with a camera.