DOWNWINDS'S PROFILE

Apologies if it's been asked many times, but I've been looking around online for ages and cannot for the life of me find an answer.

I'm in the process of editing some tilesets (nothing fancy, mostly just making the default ones nicer). On the tiles that are supposed to be partly transparent, well when I import them into Ace and try to map, the transparency doesn't 'work'... I can't explain anything. Pictures will help:


This is an example of what my tileset looks like:
(note that the background is not white, it's transparent)

And in RPGMaker VXAce:


There must be a way to make it so that partly transparent tiles don't erase whatever was underneath them. It works in the default tilesets and I've seen it work in custom ones that other people have made. I'm probably missing something stupidly obvious but it's making me really frustrated.
I'm editing in GIMP, if that's significant..