Can I design web graphics with Pages more easily?
As Pages was not really developed with producing pixel-based graphics in mind, I think you workflow is already quite the best way to handle such a task in Pages (+Preview).

However, if you have Pages, you may also have Keynote? Your task could be accomplished with this app much easier.


Slideshows are set up in pixels, not cm.
After creating a new slideshow by selecting one of the predefined sizes, these can also be completely changed to custom sizes (info window, first tab).
After finishing your artwork, you can directly export into several image formats like JPG and PNG. You can even resize the image during this step (again).


(Tested right now with Keynote 5.3)