Hi there!
It's all undocumented but you should be able to just enlarge the files and it should work.
faces.png and heads.png: add 16 pixels of width for each part you want to add and just draw it there.
base.png: 24 pixels of height and just draw it there.
Remember to use the same colors (yellow outline, green skin and so on), they are specified in config.json as fromColors