How to make a PNG image transparent

Hi there, 

I have a PNG image from the internet (an iPhone case) which I need to turn transparent. So essentially the iPhone case is empty/transparent but the background is still there.

This is for a third-party app on Shopify where customers can attach their own image to products such as an iPhone case, and the empty/transparent phone case will be filled by the customers image attached.

Please let me know if I need to explain further. 

Thanks in advance!