A fix for images that go black in iMovie

I have a fix for the problem of importing images into iMovie, and the images turn black. I bring the image up on my screen in Preview (or other graphics app) and take a screen shot of the image (Command+Shift+4). This saves the image to my hard drive as a .png file.

I rename the file so I know what it is, then I import it into iMovie. So far, it works fine every time.

I'm curious why the screenshot of the image always works fine, but when I create the image in a graphics app as a .png or .jpg file, sometimes it works, and often times it doesn't?