add placeholder for tree species/varieties without photos

[emillipede]

as a: user

when I: select a marker

I want to: see an image (or know that an image is missing not because of a glitch, but because it's not available)

• when a user selects a marker and the associated tree species isn't represented in EOL (or EOL doesn't have any media for that species), display a placeholder that indicates "no photos available"