My best try …

1. click the graphic to select it
2. go to the Insert tab on the ribbon
3. open the Shapes pulldown
4. select the shape you want, and draw it over the graphic
5. right-click on the new shape, select Format shape from the context menu and adjust the fill and outline

If you add or remove text so that the graphic moves on the page, you'll need to move the box as well. To avoid this, experiment with the More layout options (Vertical settings on the Position tab) from the context menu.
fwiw, my preference is to edit graphics in a graphics editor: work with layers turned on so you can change the position of the red box or hide it altogether, then save out as a .png or a .tif and place that in the Word file or Outlook message.