Use highlighter tool to highlight image/picture in Word

It is actually easy to hilight.

1. Select a shape, like a rectangle

2. Draw the rectangle where you want it.  Don't panic when part of the picture disappears.

3. Right click on the shape, select Format Shape (or select Format - Shape from the top)

4. Select Fill and change to a solid color, like the Standard Color Yellow.  Change the Transparency to about 75%, and click OK

5. Click on Layout, and select "In front of text"

Copyand paste this shape to the various places on the picture.

I'd suggest a shape rather than a line (as suggested by Deepa), but you'd need to set some transparency. A better approach would be to use an application dedicated to taking screen shots. I recommend SnagIt by TechSmith. Not only does it have great tools for screen capture, it has an image editor that includes a highlighter among many other tools. You can get a free, fully functional trial version from http://www.techsmith.com/download/snagit/. Before the end of the 30-day trial period, I think you'll decide it's worth the purchase price.