How do I change the default image from the cat to something else?

Agreed. :)

The process is defined in the appendix of the reference guide at http://info.scratch.mit.edu/Network_Installation

Customizing the default sprite
You can replace the default cat sprite with a sprite of your own. Your default sprite can include multiple costumes, sounds, and even scripts. To set this up, just create your sprite and export it. (To export, right-click on the sprite and choose "export this sprite."). Then rename it "default.sprite" and place it in the Costumes folder.

Hope that helps,
K