Sorry for taking so long to explain — I didn’t know, and I was working on a project and used many of their sprites. Could that bother them?
I have a mantra I work by: "build bridges, instead of burning them." We already have religion and politics to separate us; we don't need to increase the separation.
As Damon explained, we can't force people to respect other people's work, since it was made using third-party IP. But we also won't help or promote it.
My advice is: ask first.
Approach the people who made these sprites and talk to them. Most will allow you to use them, as long as you give credit (which is always welcome). If they prefer that it not be used momentarily or that it remain exclusive to their project, I would respect that and find another way to make the same sprite.
We had an example here in the community: there was a great Final Fight project, and a friend of mine was interested in it. Instead of adopting Mugen's "if it's on the internet, it's free" mentality (and I can say this because I come from that community), he approached the author and offered help, and a great update was released, and they even became friends.
But then a group came along, simply took the project for themselves to make a "mucho mejor" version and erased the original author's credits. This upset the author so much that he retired, and he was a super talented person. Only later did they start giving him credit – first subtly, as if he had "helped" with the game, and only later admitting that they had built it on his work.
But the damage was already done.