on opensuse12.1 you do not install gimp2.8
UNLESS you build SECOND and NEWER versions of things like "glib" and GTK,BABL,GEGL, png15 and a whole mess of things
that if you are not ( yes in bold and CAP ) very VERY VERY CAREFUL - and i mean VERY VERY careful will kill the Operating system if you install the newer versions of basic operating system parts in a place that the Operating system can FIND
so if you REALLY and i mean REALLY want to do this it will take some time
you will need to build the whole development tree for 2.8 outside of the OS , in a location that the OS can NOT find
in it's own build folder that is not in the system $PATH
then MANUALLY or write your OWN build scripts BY HAND
to point the gimp source to a SECOND and different $PATH, $PKG_CONFIG_PATH ,$LD_LIBRARY_PATH
------- WARNING -------
if you replace any of the suse12.1 SYSTEM PROGRAMS with new and DIFFERENT versions ( as in ONE dumb and stupid "typo" -- and they DO HAPPEN TO US ALL - you WILL KILL THE INSTALL OF 12.1 !!!!!
so do you REALLY want to do this ????????
gimp2.8 "?can?" be built on 12.1 but it is not going to be easy
But i already installed the gtk+-3.2.4 and glib-2.30.2 version.
if you installed them to /usr
it is to late
you killed your install
it is not fixable ( easily that is ) R.I.P
and reinstall 12.1
all of those NEW versions of critical system files MUST be built in it's own build folder for this project
say point EVERYTHING ( i mean EVERYTHING needed for ALL the dependencies for the dependencies to some place like ???
so that in that folder you build a complete and second operating system file structure
to put this a different way
i have been building gimp since about 2002~3 and quite often the testing development tree the odd # versions)
and building 2.8 on opensuse 12.1 is something i AM NOT going to do
gimp 2.6.12 builds just fine on 12.1