I hadn't heard of game-icons.net, that's neat! Thanks for the information.
Implementing all 1113 icons directly into the app would not be the way to go. It'd be far too time intensive to setup and would be insanely resource heavy.
Instead, create a Sprite object to be the placeholder. Then, use the action "Load image from URL". The game-icons.net images are all powers of 2, so set the placeholder sprite's size to a power of 2 as well. (64x64 or something).
Don't set the URI directly to game-icons.net images (it's bad practice, and it also wouldn't work (their Cross-Origin Resource Sharing policy does not allow it.) Instead, re-host the images yourself and point to the URI there. Alternatively, if you're using node webkit or otherwise planning an "offline" export, add the icons as project files and have the URI simply say the name of the icon in the project files. (for example, "crossbow.png")
Here's an example that uses project files: dropbox.com/s/34pisqor3m2v1uw/ebrar.capx