If you want to treat only images borders, you can't apply filter
to the whole image. The solution to get the wanted effect is as
follows:
Duplicate layer(
->
) and select it to work on it.
Apply “Tileable Blur” filter with a 20 pixels radius to
this layer.
Select all (
Ctrl-A) and
reduce selection (
->
) to create a border with the wanted width.
Delete selection with
Ctrl-K.
Merge layers with
->
|