image handling for tutorial cards

[ivirshup]

The images for tutorial cards are currently all custom links to content (mainly) from the targeted tutorials.

This is probably bad.

• The images are generated as part of a build process, and may be renamed at any time, breaking the link
• The images aren't sized to fit the cards, so there's lots of extra space

I think we should probably create custom images for each card and store those in this repo.

Either that or use a tool to auto generate the cards.