Troubles with recoloring images

I was trying to recolor the sfx torches found in Hard Doom to increase the color possibilities, but I'm falling into an issue (probably transparency ?).
The blue image was exported from Hard Doom.
The green one in the middle is a basic recoloring of the blue image (with hue variation).
I tried to add transparency in the third image to see the difference (but there is not, it applies the same with the blue image).
So it seems at a first glance that image 1 & 2 are of the same nature.
However, when I import both images in Slade3, the blank around the torch doesn't disappear on the green image, while the blue one does.
I expect this to be a novice mistake, but I would be grateful if someone could provide a solution.
Spoiler:
The blue image was exported from Hard Doom.
The green one in the middle is a basic recoloring of the blue image (with hue variation).
I tried to add transparency in the third image to see the difference (but there is not, it applies the same with the blue image).
So it seems at a first glance that image 1 & 2 are of the same nature.
However, when I import both images in Slade3, the blank around the torch doesn't disappear on the green image, while the blue one does.
I expect this to be a novice mistake, but I would be grateful if someone could provide a solution.
