allow images restyle without source key

[VeraZab]

it appears that the sources key is necessary in order to successfully call a restyle on layout.images.
http://codepen.io/veraz/pen/wWjPZp?editors=1010

Would it be possible to remove this condition as it would allow greater flexibility to create UI elements that create defaults for those other keys and are created before the user uploads an image?

[etpinard]

That's a correct.

Images with no source field are considered invalid for plotly.js as per this line from PR #525.

A way around this problem would be to have a default image source in store somewhere (e.g. in the workspace) that would fill in the source field when a user omits it.