Is there a keyboard shortcut in Gmail for the "Display images below" or "always display images from" links in emails?
Not at this time. A list of existing keyboard shortcuts can be found here.
No shortcut, but if you've previously okayed images then it should show automatically if they send something in the future with images.
If someone you've sent email at least twice sends you a message with images in it, you’ll see the image by default (because the people in this group are likely people you know and trust).