I've read the rules several times and just re-read them to make sure. Nowhere does it say that, I've had 2 pieces accepted with partial transparency, and it just wouldn't make sense. What's so terrible about making pixel art look good as intended on more than one color of background? My last avatar (a cat) had this problem. It looked fine here but terrible on the light background of the forum.  

I'm fairly certain it's against the gallery rules to use semi-transparent pixels. I'd consider removing them.