I found the first function by looking through the firefox source code first for the menuitem name of the function "Save Image As", and from there finding saveImage. The removeCurrentTab function was harder to find, but this addon provided source code that showed it: Stephen Clavering's CTC.
Tutorial pages I initially found about extension development were helpful, but I didn't see anything that talks about mozilla fundamentals- probably I need to find a book about it.
This addon goes well with the Menu Editor and Download Sort.
There is code in the real Save Image As for determining whether an image is being selected or not (my addon shows up regardless) I should add in next, and there should be logic that prevents the close action if the save as was canceled (less sure how to do that).