Add images to help cards

[johnnyquest]

Idea: put images from fb, e.g. "node of the day" pics to related nodes' help.

[johnnyquest]

I suggest full screenshots to have (roughly) the following format:

• the screenshot should be taken of an 1280x720 window (window borders disabled)
• should be resized to 50% (640x360), converted to a 256-color (indexed) png
• this results in acceptable quality and a file size of ~65k

Other notes:

• for full screenshots I (jq) don't think text readability is a concern; to provide some interesting visuals is much more important
• indexed-color (png) images are the smallest, the less colors the better...

Some example images (already added to assets):