Revert the previous commit 786686a5 and comes up with a better fix.

Let's actually change the image base type and add its colormap as close
as possible without any GUI calls in-between. I also add an explicative
comment so that people are aware of this call proximity requirement to
avoid future problems when the code gets remixed.

It should be better than hacking around with exception in GUI code, and
should (hopefully) avoid other similar bugs.