I assume the high position of Jazz in some of the screens could have been caused by float ups (seems to not be a possible explaination for one of them, though).

The ammo fired by the enemies I would consider almost impossible to make like that. Look at how big the (I'd assume) animated fires are. They'd require an extremely huge amount of animations. Some of the shots even seem to go diagonally, in different angles (are they going to home in on the player?) which requires even more animations. You'd have to put the tileset full of animations for this to work, and I wonder if you'd have enough animation slots. Even if that's possible, that seems to be bad design - it's probably possible to make challenging and visually appealing custom enemy animations without all this excess that seems out of place and just takes up space which could've been used for other things. The levels also seem to be rather.. flat. The ground's just a straight line without any slopes or obstacles, and the background is pure black.

And then there's what Cooba mentioned, the top screenshot on the other forum uses way more than 256 colours. Is Jazz X a new JJ2 client?

If you post screenshots, you should be willing to expect not only positive remarks but also criticism. If you appreciate quality you should be able to stand feedback so you can improve your levels.