Definately a very good tutorial and a nice outcome.
I tried it in GIMP and managed to do it just fine, for anyone who wants to do this in GIMP, when he says "Image -> Apply Image" go to "Edit -> Copy Visible" and then paste a new layer. Instead of "Ocean Ripple" use "Ripple" with Period set to around 30 and Amplitude around 15. For Polar Coordinates go to "Filters -> Distorts -> Polar Coords" and set both tabs to 0 and it should have a similar effect.
Where he says set layers to "Lighten" and "Darken" I just kept my layers normal and erased at various opacities because "Darken Only" and "Lighten Only" seemed to have odd effects. Oh, and to flip your layer go to "Layer -> Transform -> Flip Horizontally/Vertically".
This will definately help people Kimpen, thanks
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Here's my result:
