Add placeholder graphic for elePHPants that do not have photos

[cookieguru]

I sent the guide to a coworker that wanted to know more about the Magento elePHPant. Unfortunately we do not yet have a photo available under a CC license of said elePHPant, so that entry just has two rows of text. She mistakenly identified the True North mammoth as the Magento elePHPant since it was the nearest photo :/

This commit will add a placeholder graphic to each entry where there are no photo(s) available.

The image was created by Vincent Pontier (AKA elroubio) and is released under the GPL. That's not compatible with the CC license currently covering the content of the site, so I'm looking for suggestions on that matter.

[philipsharp]

I wonder if something else can be done with the design rather than to insert this image repeatedly. (It would be nice to have more photos, but it's proving difficult with the newer elephpants.) To get from the Magento description to the True North mammoth required reading through multiple header lines.

Do you have a source for the logo being GPL? I remember once reading it as "free to us" but I can't find that again.

[cookieguru]

https://commons.m.wikimedia.org/wiki/File:Webysther_20160423_-_Elephpant.svg