How do you insert an image placeholder in CC 2015?

The button was removed a while back.

The easiest way would be to create a quick bit of code linking to an image of your choice...

<img src="some-remote-image.jpg" alt="something pithy" />

Highlight the code and create a Snippet for it. From there on out you can assign a keyboard shortcut/keyword trigger (depending on version of DW) or double click the Snippet to add your placeholder.