(This probably belongs on netbsd-questions, not tech-pkg.  Also you
didn't mention what version of NetBSD you are running, if even NetBSD
(pkgsrc runs on many places), and the versions of gimp, gtk, glib,
etc.)

  When I use GIMP on big image files, e.g. 4000x3000 pixels TIFF (about
  75 MB), I can't do anything: at the first use of whatever filter, GIMP
  crashes with a message like: "glibc error, cannot allocate 12000 (or
  sth different) bytes of memory."

Almost certainly gimp is running out of memory.  If in bash, do
'ulimit -a', and look at max data size as well.

The default limit of 128 MB datasize seems way too small these days.

-- 
        Greg Troxel <gdt@ir.bbn.com>