fix openlayers bubbles

We introduced max-width: 100% for all img elements for a reason.
unfortunately, openlayers does some hacks to display its bubbles.

So this tries to stick to the max-width: 100% while keeping
the open layers bubbles (and possibly other hacks of that sort)
intact.
Merge pull request #653 from xi/feature-fix-openlayers-bubbles

fix openlayers bubbles