Whip yourself up some images like these In a drawing program. I made a 1024x1024 . Draw the background  medium gray 128. Draw circles 224 at least 5 pixels wide. Don't get too close to bright white 255 or to black 0. Fill the inside of the circles with a darker color to make them deeper than the 128 background. Don't make them all the same depth. Add noise by drawing around edges to make them thicker in places. Add little random dots on the surface.