To elaborate on Background Noise's suggestion:
The way to highlight an area in a picture (a scanned image is a picture) in Word is to Insert the picture into a Word document, then open the Drawing toolbar, select the Rectangle tool, create the rectangle of the appropriate size and aspect, then apply a Fill of about 80% transparency in an appropriate colour to the new rectangle.
Then simply drag the rectangle to the correct position - tweaking as required to fine-tune the fit.
Obviously this works for all the tools and formats supported in the Drawing Toolbar - lines, shapes, arrows etc.
SD