Here is the Tracker for this:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32881
You may blame the J!Tracker Application for transmitting this comment.

@radiant-tech
Sorry have not see your link here. It looks that it was lost here:
http://issues.joomla.org/tracker/joomla-cms/3208

Tested the pull request. Works as expected.

@radiant-tech The current code in com_tags is less than optimal because it converts the params object to an array. If it would be left as a JRegistry object, this PR would be so much simpler. But now we're stuck with it...
However creating a new property images which contains the original item params is a bad idea as well. If anything, create a new property params2 (or similar) and store the already merged params before they're converted to an array.
Then you could use all the joys of JRegistry and it already takes care of the menu and component parameters and you can specifiy a default value.
Otherwise just use the parameters as stored in the $item->params array. There you already have the new global value if applicable.

How do we handle the sql for adding the new config field?

You don't have to do anything in the SQL. But you need to define a default value within the code. It will act as fallback as long as the global value isn't set.

Hello @radiant-tech

Thank you for your contribution.

The last comment here was on 14th July 2014. So the question is, Is this issue/pull request still valid? Can you also have a look at the comment by @Bakual ?
If no reply is received within 4 weeks we will close this issue.

Thanks for understanding!

@radiant-tech Since no reply has been received in the last 4 weeks I am going to close the issue. If you want to continue working on it at a later date, feel free to re-open the issue. Thank you for your contribution.