PNG textures with transparency (icons) - Having trouble

Hey,

I’ve done a lot of searching and googling and this seems to be a common topic but I can’t find an answer in working-mans-English.

I’m creating item icons and I’m using PNG files as the picture for them. All PNGs without any ‘alpha’ (invisible pixels) work fine. Any PNG I import that has ‘alpha’ looks like 1) in the editor (the ‘transparent’ area around them is messed up), and loads as a white square in my game:
1)

254778-2.png

254779-3.png

I thought this was some sort of bug with PNGs in UE4. However recently I’ve bought some PNG textures from the UE4 marketplace, and they have invisible parts and they look like they load fine!

254780-1.png

What’s the difference between these two? Can someone explain to me in English how to sort this out using GIMP/Photoshop?

Thanks!!!