Hi Grammy, welcome to the forums!
When you add an image which has a background, it's like sticking a photo on to a piece of paper, everything in the background of the photo covers over what is beneath it. So if you want to stick a photo of a piano on to a collage, you need to cut the piano out of its background. Unfortunately the same is true here, so if you import a picture to stick into an ArtRage painting, there's no way in ArtRage to automatically remove that background.
There are special cases where some images have transparent backgrounds, but if you're getting this behavior, it sounds like that's not the case here.