Fix ICC color profile errors

[saulrh]

Fixes #3.

I solved this with imagemagick as suggested by https://stackoverflow.com/a/22747902: just run mogrify to rewrite the offending image with imagemagick's default output settings. This doesn't appeart to have changed any of the images, which I verified using imagemagick's compare tool. The full script that produced this commit:

bad=$(mktemp)
identify graphics/*.png 2>&1 >/dev/null \
  | grep -o "\`.*'" \
  | cut --complement -c1 \
  | rev \
  | cut --complement -c1 \
  | rev \
  >$bad
mogrify $(cat $bad)