Hi Traci,
Very easy done.
Simply open your transparent image in PDN. Then create a new layer and move it down using the "move layer down" arrow in the layer window.
Select that layer so that it is the working layer (this should happen by default when you move it down), and using the "rectangle select" tool, drag a rectangle so that it covers the two black lines. Then simply select white colour followed by the 'paint bucket' tool, and click in the rectangle.
Save your file as a .png, and you're done!
*** Note that this will merge your layers down, so rather than save, do a 'save as', and give it a new name so that you do not lose the transparency of the original image!
Cheers,
Ventor