If you weren't compensated in any way for making these, and there's no contract then yes, of course, they are yours.  The only thing borrowed is the basic character design which is too vague to be considered anyhow.

"The project leader isn't replying my mails anymore"

I've always found such behaviour beyond rude.  Even if I flake out on something, I still stay in touch with anyone who cares.  It's cowardly and disrespectful at best.

In any case, keep working on your sprites.  If you get good enough, and get enough done, I'm sure other people will take interest.  There's no shortage of "programmer looking for artist" types afterall.  It's inspiring to see how much effort and perseverance you've been putting into this little guy.

Oh, and yeah, amateur projects breaking down is VERY common.  In fact, it's the rule, not the exception.  Don't feel discouraged.

Hmm, the green guy is hard to look at.  If you could add more contrast somehow it would help.

Started on the back facing walk eh?

The height changes when one leg is forward should be the same as the height changes when the other leg is forward.  In other words, there should either be 2 "up" frames or "4", not 3.

In the up facing walk, the right foot doesn't move as much as the left one, and seems to be moving a different angle as well.  Try using the line tool to make some parallel guidelines to help you with the feet positions.

Oh yeah, one more thing... the arms are going too far behind his back in the up facing walk.  Don't just cut-paste the arm-movement from the down facing sprites.

Yes to the new cheek highlight, no to the nose shadow.  Or rather, instead of having the shadow complete the oval outline of the nose, it should darken the bottom left of the nose.

It'll be okay, as long as you make sure to claim it right away.  I've "digged up" all the pixelation posts by 1.

In the end it was good that hawken posted this "author unknown"(lol) thing so stuff could get out in the open early