You mean like this?
And you could make the blur bigger.
1. erase the background of your image.
2. duplicate the layer
3. blur the image a little, so you see a light border around the image.
4. Duplicate the original image
5. Zoom blur so you have a very very fine line around the image
6. Duplicate the original image again
7. select all of the picture, and make it way bigger than the original image.
8. Blur to whatever width you want it to be.
And then you could be done.
But if the aura is to round, and perfect...
9. on the big, blurred layer, take your eraser at about twelve for width, (something smaller if you have an especially small image) and create jagged lines around the outside of the aura.
10. Gaussian blur that layer slightly.
11. (optional) add some zoom blur.
Experiment with the values, as it will always be different with different images.