I am learning inkscape and am having trouble creating a simple cut board for an led light sign. I imported a picture that will be engraved. I just need the steps to change the graphic to a totally black image for raster engraving. Then I just need to create a box around the image, plus a small (70mm wide, about 30mm tall) under the larger box. I want these 2 boxes to be an outline image in red for cutting on my laser. I have been watching several videos and have learned much, but am requiring a little assistance on how to do this. Thank you.

if changing the image to black is the step you are having trouble with, post an image or even better the actual file with the imported picture. Not sure what you mean by "change the graphic to a totally black image" so more explanation or an example would be nice.
possibly a black and white high contrast image?