So I am developing an app that will need to use a bunch of random faces like the one attached. I just want to know if this is ok in legal standpoint and that I won't have any trouble pushing the app to appstore later.
Can I use cartoon faces for my app?
Do you own the rights to this specific image? And the original image it is based on? And are you in compliance with the software or algorithm used to generate the final image? And do you have written confirmation for all of the above?
The moral of the story is to never use someone else's work, in any context, unless you have solid legal backing. If you do not understand all of the agreements for all of the above, you should hire a competent IP attorney (not just any old lawyer) to review them.
What if I use stock pictures from sources like Unsplash?
The source of any particular image is irrelevant. All of those questions still apply. You should get a written license from a reputable stock art provider. This license will explain what you are allowed to do with the image.
Note that I am only referring to reputable stock art providers. So when you ask about a source "like Unsplash" you are essentially asking about "the internet". A significant portion of the internet operates completely illegally. Do you know which ones are legal and which ones aren't?
Even if you are specifically asking about Unsplash, you still have things to worry about. Did you read Unsplash's TLDR page or the actual license? There are some important caveats to their free license. I did a single Google search and easily found people recommending against using Unsplash due to the risk of people having uploaded illegal content to the service. In general, I would be very suspicious about any free license. A fundamental piece of written confirmation should include your paid receipt.
The best option would be to simply take your own pictures. Even then, those Unsplash caveats might still apply, depending on what picture you take.