In the new Page Manager, Pro 9.5 the highlighting feature does not allow you to highlight images (text only). However you can use the "Rectangle" tool as a workaround.
Use “Rectangle” tool for the image part of a PDF. Set the “Rectangle” tool properties to “Fill color” and set “Opacity” (50% Opacity, higher the Opacity to darker the color) to make it transparent, then draw a rectangle on the area that you want to highlight. The effect is close to the highlight. This can apply to “Oval” and “Polygon” tools too.