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.