Investigate ways of shrinking payload, particularly image screenshots

[adamschwartz]

@TeffenEllis please correct me if I’m wrong, but my current understanding is that every image for every target is included in the JS payload.

Some options to consider:

• Require devs to download and host all of the images themselves.
  • In which case we could also lazy-load the images as they’re displayed.
• Put the images on a CDN somewhere and link them.
• Serve different images for retina vs. non-retina devices.
• Continue to include them inline, but:
  • Minimize them better (JPG over PNG?).
  • Ensure we’re using the minimum dimensions necessary for each image.
  • Reuse screenshots when they are visually very similar. (For example weebly/paste-embed-code-header.png and weebly/paste-embed-code-footer.png only differ slightly.)
  • Use more SVG/HTML screenshots when possible.

cc @zackbloom