Try it with a transparent background, dark apparel only.

I've been trying the different T-shirt options and it seems to look better on a black background. But the original picture should be a bit darker to blend. The other option would be to eliminate the dark-gray background and use it as trasnparent. Bigger or thicker letters will help a lot when printing. Anyway I'm not a guru and I think you have great potential. I would totally link a product like that.

Great imagination.