Minor adjustment to image rendering in the marker pop ups

[rudokemper]

Let's make the photo fill the full width of the pop up, instead of floating to the left of the text fields (as in the below screenshot). The text can go underneath the image. Let's also ensure that image proportions are respected, not shrunk as below.

Current pop up photo rendering can be seen on the live demo: http://terrastories.herokuapp.com/en/home

Related follow up issue: https://github.com/Terrastories/terrastories/issues/350

[ohamuy]

I can do this!

[ohamuy]

I have already solved this problem if I can claim it!

[rudokemper]

@ohamuy assigned!