Using png as texture2d appears to be black

I have an inventory system kinda working now and when I try to use png pictures to represent item’s icon, the image appears to be black. I use a struct and a data table to implement my inventory. I am very new to texture and image and the unrealengine in general, so any help would be very appreciated. I have included here some pictures to further clarify what problem I am having. Note, the first image is in tga format with a white background while the rest are in png format with transparent background.