[add-art dev] Auto image update status

[Ethan Ham]

It was great to meet you on Saturday, Steve!

I've tracked down the problem we were having with the image update process.
The Americana set of images had a bunch of problems (corrupted files,
different sizes, misnamed extensions (jpeg instead of jpg).

When I made a set of images myself (I used the Japanese print set as the
basis for the new set, and then painted a green stripe on them), it worked
fine.

I checked in a fix for the problem we found on Saturday (that the extension
was using a cached version of image_set.xml).

I've noticed that the extension doesn't like the images.jar file being
changed while it is running (it stops showing the images until Firefox is
restarted).  I'm going to do a fix for that today (after a new images.jar
file is downloaded, we won't replace the old one until the new time Firefox
launches).