I'm only barely starting to learn about computer graphics. So I can't really offer the kind of comments you want. But I think it's a great image -- very nice!
I like how you put the thick, partially transparent border outside all the black strokes around the little boy. It's a really nice touch. I like how it sets the boy out against the heart.
Keeping in mind my non-pro status, there are a couple of things I notice that seem out of place. Something about the placement of the little guy's ear seems weird to me, like it's a little too close to his eyes maybe. I think I only notice it because everything else is proportioned so well.
Also, I think I'm seeing what I've heard called "pixelation" around the 'Love Boy' lettering. I'm not sure if you intend the letters to have a fine white outline, or if it's the result of some part of the process you described (the original photo, maybe)?
But what do I know?!! Mostly I think it's a wonderful image, which I'd be glad to see in any gallery! And skill to which I surely aspire.
All best
