update readme for speaker image dimensions
update readme for speaker image dimensions

Fixes #397

Former-commit-id: 1f372301fdf01cd3ab167bba4dd21654ee3fddde
benjohnson77/devopsdays-web,masteinhauser/devopsdays-web,masteinhauser/devopsdays-web,joelaha/devopsdays-web,joelaha/devopsdays-web,joelaha/devopsdays-web,masteinhauser/devopsdays-web,benjohnson77/devopsdays-web,masteinhauser/devopsdays-web,joelaha/devopsdays-web,benjohnson77/devopsdays-web,benjohnson77/devopsdays-web