Flip offset when uploading a vertically oriented avatar

Portrait images were being shifted down instead of up when being
squared off. This change reverses the width and height when
calculating offset-y to be positive rather than negative.

Addresses the sub-issue I mentioned in #457 (probably should have
been its own issue)

Before:
![profile-images-offset](https://cloud.githubusercontent.com/assets/950127/4330035/cec02b5c-3fa5-11e4-84e3-a596eba1b6ea.png)

After:
![avatar-image-offset-fixed](https://cloud.githubusercontent.com/assets/950127/4330041/e0ca4742-3fa5-11e4-8094-969510c27013.png)