Just FYI, the DPI of an image will do literally nothing to the image/texture quality, its a meaningless value as far as computer graphics go, it only has context if you start printing artwork. All that matters here is that the bump has enough pixels of resolution (~4000 should be fine for what youre trying to do), that you dont mess it up with a lossy file format like jpg (png would be a better choice), and that the artwork has some softness to the edges (gaussian blur it)