using too big images is a big problem for web apps, I experimented myself with this problem some time ago and the best solution for me was to use a black 16x16 tile I used for the background and have separate images for galaxy clouds, stars and planets that got just places onto my map (can be done either in the editor as a fixed position map or by generating the map during runtime by using random x/y coordinates.
About the zoom, you can put the planet on a separate layer that is just scaled up and down when you want to zoom. Having it on a separate layer, it will not affect the background when you play around with scaling.
For clickable, this is not a big problem, just have each planet as a separate object and when you click on it, do your stuff in the event editor
Hope this helps for the first stept