When I'm writing a plugin, do I assume that jquery is included, or should I always wp_enqueue_script it and let wordpress figure it out?
Also, what's the best practice for getting an image path figured out in my bundled css file?
One point for using
wp_enqueue_script is for that very reason. Never assume anything, enqueue jQuery so that if it is there already it does nothing, but if not , it will queue it up.
If your talking about the image path for your plugin, you can use the following, http://codex.wordpress.org/Determining_Plugin_and_Content_Directories
The examples for
plugins_url should help.