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If the Japanese writing is being inserted as a font, that is to say you typed those Japanese characters into a text box rather than copying them from an image, then a freeware application such as PaintBrush available from the Apple downloads site allows for text to overlay an image.

If the Japanese text is an image then you will need a graphics application with a lassoo or magic wand tool that will allow you to select only the black elements of the image (the text) and copy them without a background.
 
If the Japanese text is an image then you will need a graphics application with a lassoo or magic wand tool that will allow you to select only the black elements of the image (the text) and copy them without a background.

Yes, use the magic wand tool to select the white space and hit delete. I recommend PixelMator or GraphicConverter to get this job done.
 
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