Mosaico provides a tool for generating thumbnails of the blocks.
The following worked with node 8.11.3, yarn 1.7.0
git clone git@github.com:veda-consulting/uk.co.vedaconsulting.mosaico.git
git clone -b "v0.15-civicrm-2" 'https://github.com/civicrm/mosaico' packages/mosaico # ./bin/setup.sh -D does this but with npm
cd packages/mosaico
yarn install # takes ages.
yarn add grunt-cli
# You can now check makeThumbs works:
# yarn run grunt makeThumbs:main:versafix-1
git clone git@github.com:pbatroff/giga_template.git giga_template
cd templates; ln -s ../giga_template/versafix-giga ; cd -
# We can now build the thumbs in our custom template with:
yarn run grunt makeThumbs:main:versafix-giga
# Copy to server with:
rsync -arv --exclude=".git" giga_template/versafix-giga/ giga:pro/sites/default/files/civicrm/mosaico_tpl/versafix-giga/