It should be possible to add the assets from outside (upload) from users exisitng element libraries, and not only create them in the app as described, for many it could save time building the own asset library:)
Randy Fournier commented
You can, just drag your image into your open project window...then just resize it to your liking and then you can right-click on it and add as asset.
If not, at least the ability to copy paste elements from one mock to another.
I too would really like to have a common pool of assets visible in all mocks. Currently I have to recreate objects :(