Drag and drop advice

First of all thank you so much for such a wonderful material. 
I am really greateful how I can give you back for such enormous gift.

i just joined the community and I take advantage to greet everyone in here.

now passing to my topic just  a quick question I have created a content I  drag and drop but I can't remove the background. to be more specific.

I have created a tree where i put apple and my son has to drag and drop them into a basket. but the apples they have a white background as a pictures. not like a fruit in the tree.

I have converted the apples pic into PGN version but it doesn't acepto just JPG. 
but my curiosity is that in the demo there is a chess game and the pictures they don't have that backgound.

can please anyone help me.

thank you so much again