I have been looking for a possible answer for this problem (it seems that I am not the only one facing this), but the answers do not help too much.
I got this image (.png) which I consider having a good quality even when I see it in lyx. But as soon as I create the pdf (using pdflatex) the resolution of the image decrease as you can see.
I got not idea about why this is happening, almost everyone in other answers says that .png images should appear like they are.
Do you have any idea about this?