BluePhaze, thanks for your suggestion. I'm using inkScape for my animations but I'll take a look at Spriter to compare them. I will take in mind your suggestion, I'll keep working on my animations for jumping and falling and I'll let you know how's it going.
Now I don't know why when I put the background image and I move the character (left or right arrow), the game scene seems to shake and it's a very annoyng effect.
I uploaded the version of the published game if you need to understand what I'm talking about (http://ec2-54-227-40-240.compute-1.amazonaws.com:50000).
Thank you very much.