You do not want a higher scanspeed then 30kpps
For graphics in general scanning higher then 30kpps is not needed and not recommended.
Your corners will look better though when your scanning at a lower angle and keeping your scanspeed at 30kpps
Just do not scan too small and with 200mw you'll actually dont have to worry too much at all, unless you have a static beam.
Great tread, hopr you will get a great show
Panolin Quickshow 2 and Beyond whit 2 FB3 controllers, full RGB
Feel free to join my Laser freak group on Facebook
http://www.facebook.com/#/group.php?gid=53680012853
Make sure the balloon is anchored to the ground and have enough bleed along the edges to allow the balloon to move/sway in the wind and still have the image on the cloth.
It doesn't need to be perfectly centered, but at least make sure the image won't be trimmed at the edges when the balloon moves too much, which may require you to project a slightly smaller image.