Just for the sake of discussion, I (also 5'7"-5'8") might have gone for the 56cm as well, but with an 80-100mm stem and 170mm cranks since that's how my bikes tend to be set up. I think it works out about the same.

As for you, it looks good to me -- leg is slightly bent at the bottom of the stroke, knee is over pedal pedal spindle, etc. How does it feel to ride?