How to stretch the edge of a picture.

[enginestar]

Say I crop a picture.
On the left I have an edge that is 5 pixels - after my object.
I want to have a border around my object be 20 pixels say.

How can I stretch the edge and make it 20 pixels?
I used to do this in Photoshop.

As an example see the image below


Say I want to stretch the left hand side.
Ignore the shadows! This is just an image I took off the internet to help ask my question!

Thanks.