Ah! That's what was bothering me about the animation, Yeoman figured it out.
If anything, the arm in the back should probably go further (technically, further in X axis) than the other arm, due to the angle, no?
Also, I've never had to face the problems of using an outline-less sprite on multiple backgrounds myself. But of course I think the most important thing to do would be to avoid putting him on a background of the same color at all costs, so a similar brown background in any of your stages would probably be bad, or need to be carefully designed.
Looking good, though.