This can be debated for all eternity of course, but in my opinion, pixel art isn't "posterising a low-resolution image".
And, if you look at the Pixel Joint Pixel Art Primer, they don't think it is either.
https://pixeljoint.com/forum/forum_posts.asp?TID=11299As you're trying to get accepted on their gallery, you need to play by their definition, no matter what your own opinion is.
So, to make this into something you can submit, you will have to produce a piece where it looks like you have exerted control over every pixel in it. In the current version, there's lots that's left over from the posterisation process.
What I normally do is use the source image to trace blocks of solid colours over significant areas. Then, once the original is completely covered or deleted, I can apply pixel art techniques to re-draw the image. I don't want any of the pixels from the original in the final work.
I'll use the original as a reference, of course.
That way I end up with a cleaner, more stylised work. One that looks deliberately made, rather than emerging from the noise of a low-res photo.
I'm not saying there isn't a place for the kind of process you used. It's just that if you want to call it pixel art and get on Pixel Joint, you'll have to put in the graft.
If you want help with specific techniques, or just help with where to start, we're here.