Sorry my mistake for some reason I was thinking the "" would grab everything up through the last directory. But it only gets everything up to the current directory the .htasccess file is in, and for most people it is the primary web directory.
Try this instead
RewriteRule ^\.a/(.*)?$ images/$1.jpg
note the "\" it is there to escape the period (.) so don't replace it with "/"
Also do a search for "apache mod rewrite regular expression examples" I found 3 good sites in the first 10 that look to do a good job of explaining the little details of mod rewrite and regular expressions.