My example has been made with Photoshop
first i did a .png with a sized accepted with the game , e.g. 32,64,128,256,512,1024....
i ve took here the text tool and i written " Hollywood" (you can paste any copy of plants, flowers, or ect if you want)
as you can see in screenshot 1 there is no background. if you copy pasted some flowers, you can select and delete all the unwanted background pixels till you only get the leaves you want.
sorry if my photoshop is in french version...
screenshot 2 shows you : you go click on "layers" (named "couches" in my french version)
and click "create a layer" (named "creer une couche" in french) in the icons at bottom right of the screen (screenshot 2)
you will see appear a new layer named Alpha 1 (screenshot 3)
then you go in the "calc" TAB and you control + click on the little square representing your original
and some selection draws will appear on your alpha1 layer (screenshot 4)
you need to take the pencil and white color and "paint" everything in white. if your "selection draws" are made ok, you should only paint in white the zones you really need (here "hollywood" but if you pasted some flowers, you should only see your flowers in white) (screenshot 5)
then only left to save it . in screenshot 6 you see a french version of the menu , you will pick up "Save for web...." (screenshot 6)
and then select PNG-24 and check the "transparency box"
and click save at the bottom of the window and give it a name.png (screenshot 7)
and then your .png is ready for be imported in Banished and will be compatible