Make sure you draw the background first, and then the highlight. I would refrain from using org.newdawnsoftware.slick.Graphics to draw images as it is not needed but that is just a word from the wise.
It's just because I prefer to write Graphics.drawImage() (programming legacy with Java graphics). But if you tell me that is slower than Image.draw(), I will reconsider it.
EDIT: are you trying to achieve a highlighted affect
That's it. I want to make an effect like the Photoshop additive layer mode over the landscape behind.
If you want to use additive blending mode then you need to make the areas you don't want to modify black (in your additive texture).
I see ...
I thought that the pixels with an alpha value equal to zero would not affect the drawing.
So I have to add a black background to the highlight image, I'm right ?
EDIT : I have just tried it and it works. Thank you !
But could you explain a little more the fact that I have to put a black background ?