This script has been changed to provide Shelling out (calling) many different programs to provide functions not supported by Gimp.
Newest version adds the ability to specify the extension of the temp file to use, i.e. png, bmp, jpg.
Latest script here: viewtopic.php?f=9&t=970&p=17957#p17957
After reading the comments on PSPI and the issues with 2.6.11 I checked my own set, and discovered that many of mine don't work either
(shows how little I use them, oh well).
So I ran those I could through the Gimp User Filter converters into .guf file, great, but a bunch still were no-go.
Reading about XNView's support made me think, so I modified Michael Munzert's LABcurves python shell script http://www.mm-log.com/lab-curves-gimp
to call xnview.
This is python, so you do need that. Just extract the file into your plugin folder and modify the line:
path = "c:\\PF\\XnView\\"
to reflect where you installed XnView. Mine is in c:\PF but most would be under Program Files, so you WILL have to edit this to get it to work.
It shoes up under Filters->XnView...
It should respect selections, transparency, and layer masks. It saves the file as a temp png, launches XnView opeining that file. Do what you want, then close XnView, saving and overwriting the file. When the plugin sees XnView close, it loads the modified file back in.