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)
Merge pull request #469 from kylewm/avatar-offset

Flip offset when uploading a vertically oriented avatar