Select Tools > Material Base. On the left hand column highlight "Graphics: Characters", and then select the Import button. Find the files you wish to import, and highlight them, select okay.
You will be presented with a screen of your resource, you will have two boxes below it to show you your transparent color and Semi-Transparent color. Left click on background color, to set it. This color will now be transparent. Right clicking on a color will set it to a semi-transparent, which is mostly used for shadows and the like. If your graphic is poorly done, or has antialiasing, you will have to clean it up using an art program so these two sections (background, or anything semi-transparent), are a unified color.