Photoshop :: Selection Expansion?

I am currently undertaking a project creating A1 posters displaying numerous archive pictures from of where I work.

I have got the poster design etc sorted and all is going well.

To each archive pic I'm using I am adding a small off-white border much as a lot of old photo's have. The method I have been using to do this is simple. I have created a large document with my off-white paper texture covering the whole area, using this document I perform the following steps

1) drag archive pic into my off-white document

2) create a selection slightly bigger all round than the pic

3) copy the background and paste it to a new layer

4) link the new layer to the pic layer

5) merge the linked layers and rename resulting layer suitably.

This technique works perfectly for what I want though is somewhat time consuming. As such I have created an action so all I have to do is drag each archive pic into my document and run the action.

This is where I am having a bit of a problem. as each archive pic is a different size I needed to think of an automated way to make the selection I desired. To do this I used the following steps in the action:

1) using the magic wand, select the transparent area of the archive pic layer, thus selecting everything except the pic

2) invert the selection so the pic is selected

3) expand the selection by 16 pixels

4) repeated step 3 (this is due to the size of the pics I'm using.

This works ok, but I want sharp corners, but when using the expand selection tool, the corners become rounded off, can anyone think of a way around this?

At the end of the day I can use the action very quickly put a border round each pic and then I'll just have to either crop them so the corners are sharp or something, which won't take too long, but it would be nice to not have to do this.