My images in preview is too dark

Which image mode are you using. You have “shade according to power” set, so what power are you using? You didn’t answer my questions you see!

I get the above in “Newsprint” …

and the above in “greyscale”…

so its important to understand image modes for your engraving:
https://lightburnsoftware.github.io/NewDocs/CutSettings-Image.html#image-mode