Keep asking yourself: what will happen when the laser burns? After I am done, will the laser-engraved graphics be white, silver, dark brown, or some other color?
For acrylic, when the laser fires, the engraved image will end up being a whitish frosted image. For wooden plaques, when the laser burns, the engraved graphics will likely be a darker image. If you are applying color to the photo, what color is the paint? Will that paint be a dark color or a light color?
There are many questions that need to be answered, but there is a simple way of deciding whether you engrave using a positive or negative image. Usually, if the engraved graphics will be darker than the surrounding area, I will use a positive image. If the engraved area is lighter than the surrounding area, I usually use a negative image.
—Richard Korbyl, Columbia Awards