Trying to improve zip size

Hi,
I’m not sure where else to post this question. So hopefully I’m in the right place.
Anyway here is the deal, I’m one of the creator of the plugin:

http://wordpress.org/extend/plugins/wysija-newsletters/screenshots/

I’m trying to improve the size of the package which was about 2.3Mo. I know that this size can be a problem on a couple of hosts, and make it more complicated for site owners to install the plugin.
The biggest improvement to the size that could be made to that package would be to remove the screenshots from the zip. To be honest I don’t see any reason to have the screenshots in the zip nor the readme.txt file.

So my question is: Is there a way to tell the wordpress robot which generates the zip to don’t include the screenshots and readme in the zip ? This way the package would go down to 1.7Mo ! So much better!

And then if that’s not possible as I can imagine.
Can you explain me why when I changed the format of my screenshots from png to jpg to improve the package’s weight. How come they don’t show on my screenshot page ? http://wordpress.org/extend/plugins/wysija-newsletters/screenshots/

Even though they are here :
http://s.wordpress.org/extend/plugins/wysija-newsletters/screenshot-1.jpg
http://s.wordpress.org/extend/plugins/wysija-newsletters/screenshot-2.jpg
etc…

Thanks in advance 🙂

Ben