Graphics: autoadjustment of element colour for background

Hi. I want to annotate places on a photo (panoramic view) with labels set up in the sky and vertical lines down from the label to the place it describes. Like the tables you see often installed at scenic panorama points, explaining what you are seeing out there. Trouble is, the vertical lines cross both light (sky) and dark (ground) areas of the photo, and whatever colour you set them, if they're visible on one kind of background, they're invisible on the other.

Is there a colour setting that automatically makes the line black against a light background and white against a dark one, or something similar?

Probably something similar exists in the more detailed, professional apps like PhotoShop or similar, but I'm looking for something more homegrown like IrfanView or PowerPoint. I'm no expert, and this is only for website use anyway, so high-res isn't important - simplicity is.

Thanks for views.