Putting an image above an image

So im a primary student that just started to use coreldraw for my task of making a design of a food packaging. So there's 2 image, one is a background and one is a nugget pict with transparent background that i downloaded from google. I put the nugget pict above the background pict, then i save the file as a PNG. When i open the file, the nugget pict still has the transparent background shown, which is a mosaic background that covers the first background image. Can someone help me how to get rid of this mosaic transparent background?