Yeah, I feel your pain. Everything unreal is a little different and confusing. I've started these tutorials to try make sure people don't go through the same pain I did
What programme did you use to create the png? Sometimes a different programme can use an export method not recognised by unreal (I know that sounds stupid but I've learned that a lot of things about unreal are stupid). If you are able to open it up in another programme, or a web basses image editing package, and re export that may help
