Middleman Simple Thumbnailer 
Middleman Simple Thumbnailer is a Middleman extension that allows you to create image thumbnails by providing resize_to option to image_tag helper.
Installation
Put this line into your Gemfile:
gem 'middleman-simple-thumbnailer'
Usage
Enable the extension in config.rb:
activate :middleman_simple_thumbnailer
And modify your image_tag's by adding resize_to parameter:
= image_tag image, resize_to: '50x50', class: 'thumbnail'
Build/Development modes
During development thumbnails will be created on fly and presented as a base64 strings. During build thumbnails will be created as normal files and stored in same dir as their originals.
LICENSE
MIT