Device screenshot images too big for content

[hollyschinsky]

The sizing of the device images is too large for the docs content if left at full size.

Below is a screenshot from an Android Nexus 6 device using basic image markdown:

And a screenshot from an iPhone 6 device in the content using basic image markdown:

However, here's how a desktop screenshot image appears

[GarthDB]

I added styling for .mobile-image. I'm assigning this back to you @hollyschinsky to apply the class to the images you think need it; feel free to close this when it's done.