Animating falling leaves effect

Start by opening an image of trees.

Create selection of a leaf using polygon lasso tool.

Press Ctrl+J to copy paste the selection in a new layer. Create a duplicate of the leaf layer and move it a few pixels downwards.
Create more duplicate layers, rotate and place below each other in a random fashion. Select all leaf layers and press Ctrl+G to put the layers in a group.
Similarly create selection of other leaves and create duplicate layer falling on the ground.

Open animation window. For the first frame hide all leaves layer.
Duplicate frame. Start unhiding the leaf layers randomly.

 

Reaching towards the final frame have more leaves near the ground.
For the final frame hide all leaf layers.
Press Ctrl+Alt+Shift+S to save an optimized animated GIF.