Try to fix the 'images-resize` task's handling of :out-dir.

[wiseman]

Fixes #233.

I think this makes images-resize's behavior more consistent with other tasks with regard to putting output files in a location corresponding to their original locations. Specifically, this keeps resized images next to the original images, unless :out-dir is used.

I'm guessing there might be a better way of doing this, since other tasks can have a default :out-dir "public" and this uses a default :out-dir nil.