I have an app where I allow the user to crop areas out of a picturebox (same function as Microsoft Photo Editor). The user draws a rectangle which can also be resized or removed once drawn.

At present that rectangle is red to make it show up, but if someone was editing a red image, it would be hard to see. How can I make a line appear to twinkle, so it will always show up on all backgrounds?

The crop box in MS Photo Editor does the effect Im trying to achieve.