Basic UV Mapping

Prepare Object.

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.