This code chunk is repeated 3x that I can see.  Might it be worth making a utility that does this, so that if someone wants to change this they only have to identify one location in the code?  i.e., this could be something like
```
config['image_origin'] = return_origin_position(params.get('index_convention','1'))
```
where ```return_origin_position``` compares against the various allowed conventions, raises an AttributeError if necessary, and returns a PositionI?