You need photos with a contrast of colors or shades, so that will be reflected on your laser engraving. Are you using the PicLaser image editor? If so, the Preview Gray Scale selection will give you a general idea how it will laser engrave with your EmBlaser.
However, the overall darkness/lightness will be determined by the feedrate setting and it needs to be set based on each type of material your engraving on. I always sand across the grains of wood with fine sandpaper first. Not all types of wood burns even close to linear either. I have the best results with minimal grain woods like Birch Ply, Poplar (without any green coloring) or Basswood.
I always use a 255 (full power) for the Max Laser Value and adjust the feedrate until the darkest shades engrave to my liking, then adjust the Min Laser Value to where the lightest shades just start to lightly burn.
Here is some instructions I did for our PicLaser Lite that was included in the LazerBlade (EmBlaser) KickStarter Campaign. http://forum.darklylabs.com/index.php?p ... test-image
PicLaser is generally the same program with the Image editor that we added later. There is an image example to test with there. The original and after editing. Depending on your EmBlaser's Laser wattage and the material your engraving on, you will have to adjust the Pixel Resolution (burn line step over), feedrate and the Min Laser Value.