It may sound terribly stupid, but I have been looking on Google, on the Powder Toy itself, trying to figure out where the upload button is. Please help, and don't bash on me if I'm doing something seriously wrong :p
Also, make sure to save a page while it is paused, so that way you won't accidentally try and save a bomb a few frames after detonation. And use the stamps section a lot, it is basically a private save zone that holds all of your stuff you wish to be privately saved. It can be activated by pressing "s", then Ctrl+drag from top left to bottom right of an area on a screen to save a selected area of stuff you have built. Then press "k" to open up the list of stamps. Scroll down if you have more than one page of stamps.