Instructions for this image:

1.  Draw gradient.  (Ctrl - J)   Default colors.  Radial2 Gradient.  256 x 256.

2.  SineWave L/R 2 peaks 128 height.   (Ctrl - E)

3.  Rotate90 (Ctrl - Alt - X)

4.  SineWave Both UD 6 peaks 90 height, LR 2 peaks 128 height.  (Ctrl - X)

5.  Copy Image to Buffer.  (Alt > B > C)

6.  Hidden effect:  click anywhere on the window, outside of the image, to produce a multi-color blob.

7.  Edge merge 1/4.  (Alt > I > S > M > 4)

8.  SineWave Both 6 90 2 128 (same settings as before).  (Ctrl - X)

9.  SineWave U/D 6 90.  (Ctrl - D)

10. Repeat steps 8 and 9 a total of 4 times.

11. Gaussian Soft several times (about 10).   (Ctrl - Alt - G)

12. Color Rotate 8 bits (repeat twice).  (Ctrl - O)

13. Lighten (repeat twice).  (Ctrl - Alt - T)

14. Marble (use defaults).  (Ctrl - F3)

15. Negative.  (Ctrl - N)

16. Map Texture, default brightness.  (Ctrl - F2).  Use Buffer.

Note that this will not be an exact reproduction, because the marble effect is a random fractal.