How to feather to invisible

PSE9 under MAC OS 10.6.8.

I don't know how I can add to what the title already says.  I would like a photo to have a soft border that feathers to invisible.  I've tried to set an absolute invisible border (select invert followed by a background erase of the selection) and feathering that, but I inevitably get feathering that starts at the outer edge, crosses over into the photo itself leaving a sharp edge showing, and extends too far into the photo.

I simply want  all four of the photo's edges to "disappear" into nothing. 

Any help?