PNG files in front of background

ok, i've been searching all over "Forums" and "Answers" but cant find what i looking for..

basically i just wanna ask how to import png files into a scene? because everytime i drag it from materials, it always show the white backgrounds.. i confused..

Here is what i do, i make a plane for background, and i attach it with sky image..

but i want to place some grass, or trees, or mountains picture in front of the background, and don't know how to do it.. i've been searching some tutorial on youtube, but some says must use Umotion to do that, but in some videos, they just drag the tree object and place it where ever they want.. should i play with layering? or just drag drop the object?

could someone help me with this please? Thanks before.