I like the bit of smudge on that one. especially in unison with the light on his shoulder. too much black present in the sig is the biggest issue for me, though.
I don't know much about gimp cause im a ps user so the only tips I'd be able to offer are probably through cnc
if you have them, try gradient maps. they're a great finishing touch adjustment especially on blend modes etc.
oh yeah, try creating some depth too. blurring bits like the bg, crawling into a bit of the bubbles, can help you achieve it.