Re: your first image showing "black" render, what I'm seeing is the leaf image surrounded by a black border. Which tells me you don't have a mask for you leaf image. So what you need to do is:
- make a mask (same format, eg PNG, GIF or what ever as your original leaf)
- load the plant object into terragen
- on the left side of the screen highlight the object name, then select the "surface shader" tab beneath it. click the box to the righ, and select "go to parts shader...."
- the parts shader box opens up. click the box to "edit the internal node of this network"
- There will be one or more red box/grey box pairings. Double-click the red box that corresponds to your leaf image.
- Another box opens up. select the "opacity" tab, then click the Open FOlder Icon on the "opacity image"line. Navigate to the folder containing your mask and load that mask.
Hopefully you'll now be able to render your image and see the leaves as they should be.
If that doesn't work, try cursing, threating the computer or banging your fist against the wall. Won't fix anything but sometimes that makes me feel better.
I know that images have an Alpha Chanel where you can load the mask and bypass the exercise above. But they don't much seem to work in Terragen, so I usually end up gong through the exercise above