it's very simple to do this, and this can work with any program. (More or less)
(this isn't how greenscreening works. but this is how i did it.)
1) Enable alpha channel. this will enable transparency.
2) Put a extra layer under the stock image (This one)
3) Select the greenspace with a magic-wand tool. (make sure it's the square select. the normal wand tool wont work)
4) Delete it.
5) Deselect the area by just clicking out of the image boundries. (this will only work if you zoomed out the picture enough to see the background of your image manipulation program of your choosing)
6) add whatever the hell you want!
This should work for you, I used gimp to draw the one seen here on this thread. hopefully this helped you.
How NOT to green-screen^
How to ACTUALLY green-screen:
in your given program of choice, whether photoshop, paint, etc, look at the toolbar area of your workspace.
Look for "select," click that and the within the drop down should be "select color" or something along those lines (some software does not have this option, if not, then you actually have to do it the way Na pulse did it). You will either be prompted, or you will have a tool of some sort to select a color.
My experience with photoshop, it's select > color select > click the color on the screen, you may have to modify the differentiation
paint, no idea if it has a color select you're on your own lol
