Just add a picture of the Goo Balls you're changing, or of a level that looks interesting. For a Mod the screenshot isn't as important as for a Level.

But it just changed sounds. Not ball's image.

Correct. You'll still have to upload a screenshot for us to publish, it, however, so an image of the Goo Balls you're changing, or an original level, will suffice. Or you can make your own image, like Pavke did for Piano Music.