There is no icon in wbar after installed gimp2-basic.scm
I think the error is in /apps/gimp2-basic/share/applications/gimp2-basic.desktop
the last line is:
X-FullPathIcon=/apps/gimp2-basic/share/pixmaps/gimp2-basicpng
and should be
X-FullPathIcon=/apps/gimp2-basic/share/pixmaps/gimp2-basic.png