Transparent Boxset Poster

Hey Guys.

I changed from mediabrowser to XBMC. But now i have a problem i don't get solved.
I want some custom boxset posters for my movie sets. Diese hier
The Problem is that they dont get transparent. I get a black background on all of this. The posters are gif's.

Thanks in advance...

PS: Sorry for my eventually bad english, but i hope you know what i mean.