From the looks I’m guessing you are doing a negative image on a tile. I’m having an issue with it being darker on the sides. If your doing negatives, then it’s actually burning too much on the sides, like my issue.
Try engraving the same image with Laserbox. I suspect it will engrave evenly. When I engrave photo images I use Laserbox because Lightburn can’t handle basic png or jpg very well.