Okay, when your importing the tileset into the project, make both the Transeperant color and semi-transperant into white.
If that doesn't work, then open up Photoshop/MS Paint/Any layer based program, and use the magic eraser (Photoshop tool) to remove the whole background. Since some of the white maybe has a different white.
@ShellShocked: If it were in the first layer, the surrounding would be lighter not dim.