|Go to edit mode ("TAB key").|
Then press select all faces ("a" key).
Then pree "u" key ti unwrap.
|Go to "UV Editing" Layout.|
|Select the object if not selected, go to edit mode (right pane).|
You can see the unwrapped UV on the left pane.
Open image at the left pane (bottom). Image > Open.
Select an image file.
|Use the mouse right click and grab "G" key|
..to adjust the UV vertices on the left pane.
|Done! Go to "Default" Layout and activate "Textured" mode.|
|That's all folks. Here I'm using a small icosahedron with hair particles.|
Attach the single hair object/group as a leaf.
Use "Convert" to make it usable in the game mode.