How to add DOF afterwards to a Mandelbulb 3D image with Z buffer?

[schizo]

Heyo,
when rendering larger images with Mandelbulb 3D, then I often have out of memory problems when it comes to the DOF calculation. DOF can't be rendered it big render mode. 
So I am looking for a free program where I can add the DOF using the Mandelbulb image and the Z buffer image.

Any tips or tutorials?

[ellarien]

I've done it in Photoshop Elements, not for memory reasons but because the DOF calculation was taking a very long time for one particular family of fractals. It may be possible to reproduce the steps in GIMP or similar.

What I did was to use the Z-buffer as a layer to select the area I wanted out of focus, and then applied Gaussian blur to that area in the main layer. One time I even used two different levels of blurring on different gray levels in the z-buffer image. It isn't quite as good as the real thing, and it can look a bit odd around the edges of the in-focus object if you look closely, but it's a decent approximation.

http://fav.me/d834j7h was done this way: http://fav.me/d854etq was done with within Mb3d (and took hours!)