Overlapping pictureboxes?

I am making a game, but I can't figure out how to put one image over the other. The background of the front image, covers up the picturebox under it.

For example, I have two fish images, but when one is in front of the other, its background covers up the other fish.

I attached a picture as an example of the problem. What's the easiest way to fix this?

Thanks.

You didn't attach any files. Or maybe can't attach any files until a certain number of posts?

Anyway, I don't even know what programming language you're using. Be specific.