Technically its not illegal.
Technically, it's a
civil offense in the U.S., and a criminal one in many other nations. In some, it's legal, but it shows a blatant lack of respect regardless.
If someone doesn't respond to your question, you can see this as: hey, they didn't say I couldn't.
That justification falls entirely flat. If you get no response, or the license isn't specified, then by default you
can't use it. (Again, this is how copyright law works in most, not all, nations.)
Let's say you contact an artist, asking to use their work; you wait a week for a response, and don't get one. So you go ahead and use it, because of your justification above. And now after your KS level has been around for another week, the artist finds out. Turns out he was on a two-week vacation and didn't have access to his email/other contact method. Turns out he's pretty ticked about your level, too, and since he's not a jerk he doesn't sue but he does get LPChip to take down your level. Now one person is upset, one is depressed, and one is annoyed.
As opposed to the option of NOT using said content, which, at worst, will result in the loss of one KS level - which probably would have been a pretty mediocre one anyway, what with using that other game's graphics and all.
That's almost a worst-case scenario, of course, and it's much more likely that the artist would never find out. It's still illegal, however. Were I a mod, I would delete that level as soon as I found it, and warn its author about doing such things in the future. But I'm not a mod, because of things like that: I'd enforce the Forum Rules instead of the LPChip Rules. Nobody wants that. Well, except me.