Just defining a new file name.
( The old one with an affix like (Medium|Large|Small|Custom) before the file extension )
They are -- but they will be global variables and you'll know from your programming that it's good practice to use the narrowest scope wherever possible. Hence Local_a and similar.
Having to use a Variable token in a text field is another matter. Yes, it has the unfortunate effect of filling text fields with %Variable% soup, but it does have the advantage of making it explicit that "here's a variable".
On the bright side -- we don't get over-stimulated by single- or double-quotes wrapped round every bit of literal text, nor have to deal with their different behaviour in different contexts, special interpolation incantations, etc, etc!