i highly recommend you Gimp. Its free and you have basically all options for simple aor advanced spriting.
I like what you did here. Muscles are well drawn and positioned.
I'd like to direct you on light source problem. It's impossible at the moment.
Look at the shoulders.
If we are going from left shoulder to the right shoulder you have...
(left shoulder)
SHADOW HIGHLIGHT SHADOW HIGHLIGHT (right shoulder)
HIGHLIGHT SHADOW HIGHLIGHT SHADOW
Thats wrong! HIGHLIGHTS are must appear only in the right OR only in the left, it depends on which side is light source(if there is only one lights source)
Lets assume that light source(lamp or Sun) is in the right!
This how it should look like:
(left shoulder)
SHADOW HIGHLIGHT SHADOW HIGHLIGHT (right shoulder)
SHADOW HIGHLIGHT SHADOW HIGHLIGHT
If you don't understand what i am talking about, check out few youtube tutorials, ... like this:
click me
I'll edit this post with concrete editions on your drawing in awhile.