Different border size for each side of the image

Some quick notes:
- def border(image, all, left, right, top, bottom, colour, alpha)
- all is added to every side, so the left border is all+left, and so on ...
- I would first do the top and bottom border (=boxes) and then left and right, so you avoid overlap (which is an issue for transparency)

Please add conditional showing fields with the get_relevant_field_labels() method. See save.py as an example and read here the last note on fields: http://photobatch.wikidot.com/writing-actions#toc13

You should add as a first field but add _t():
METHODS = ['Fixed','Custom'] #feel free for a better formulation
fields['Method'] = self.Choicefield(METHODS[0],choices=METHODS)

Fixed -> All
Custom -> Left, Top, Down, Right

(I might even myself add a condition to hide the opacity field when all values are positive.)