Highlighting map area

I have imported my map animation segment successfully. For a portion of my movie, a major fwy next to my subject travel animation is an important feature to highlight to the viewer. Is there a way to emphasize an area like a freeway with a static overlay? I tried tracing the freeway over an image of the map and exporting it as a transparent image, and selecting it as the custom image for a symbol. That was successful, but the imported image was too small. I essentially want to import a transparent image, position it on the map with the ability to adjust its size and lock it in place. Any ideas?

Thank you!

Alex