Change default positioning of SVG background image.

This is, frustratingly, hard-wired at xMinYMin (i.e. top left) in Sketchily with a TODO note in the SVG editor that it should be "handled properly" at some point. Until that point arrives, we'll have to hard-wire it the way we want.

https://www.pivotaltracker.com/story/show/59358426 [#59358426]