The system unload action are for memory management and clearing textures that are not currently in use from vram. As started in the manual, this can only be done of there are no instances of the object remaining on the screen and on the tick after the last instance had been destroyed.
If the object is recreated after being destroyed and unloaded, it will load up the whatever object textures it had before from memory (not vram), including animation frames that were loaded from url before.
The way to clear the frames, as I mentioned before, is to use the load image from url again, loading an empty image, to overwrite and replace whatever you had previously loaded.
What is a guess is what you have already set up, which is unclear to me, so sorry if I misunderstood your question, which I understood to be "How do I clear the frames loaded with a URL?"
Here's an example dropbox.com/s/j2nm4ocxe0w1163/memorymanagementexample.c3p
Thanks for that, but it doesn't help my situation. What you have there is it loading one image if you press one button, and it loading another image if you press another button. It doesn't matter that the button is labelled "clear", it's still loading another sprite.
In my game, if someone called Bob plays, it will download bob.png from my server. If someone called Mary plays, it will try to download mary.png from the server. However, there is no mary.png, so I want it to instead load the default object sprite. For some reason, Construct doesn't allow this... So if bob.png has already been assigned to frame 0, anyone who plays afterwards on frame 0 will load up bob.png.
When you said "a blank one" I thought you meant a blank URL, not a blank sprite image. However, if I load a blank sprite, it will be blank, and will not revert back to the animation already loaded to the sprite.
For now, I can get around this by having the default sprite in my files list, and having construct load that BEFORE trying to download it from the server.
This feels like an unnecessary step, and so I will put this forward as a feature request.