How do I remove these images by compiling gimp from source?
I can't find any file containing these images.
They are hiding in /app/widgets/gimpcairo.c
at line 140 to the end of the file.
You should be able to comment out the contents of those functions.
If you want to get rid of the space where it uses to draw the toolbox one you can look at these lines.
gimprc
has a property calledtoolbox-wilber
which you can set tono
to remove the mascot from the top of the toolbox. Perhaps you can grep the source tree for that and gain some insight or even discover an undocumented config option ;)