I made a plugin so that you can resize images in markdown specifically because of this thread.
The syntax is based loosely on the way iOS handles multple image sizes combined with normal markdown image embedding:

[white house](http://cl.ly/image/2L3F1M2E1X3l/IMG_0136.JPG@500x)
[white house](http://cl.ly/image/2L3F1M2E1X3l/IMG_0136.JPG@x375)
[white house](http://cl.ly/image/2L3F1M2E1X3l/IMG_0136.JPG@50%)
[white house](http://cl.ly/image/2L3F1M2E1X3l/
[email protected])
Install via npm:
npm install nodebb-plugin-image-sizer
Source:
https://github.com/Kern--/nodebb-plugin-image-sizer
Known Issues:
Images that are set to be wider than their container become elongated (width is set to match the container, but height is not adjusted). This means that mobile can get a bit dicey.