Grid posters with transparency

I want to use some posters (SDPosterURL, HDPosterURL) on a grid screen that have some alpha transparency. When the grid screen renders, however, the transparent bits show up black instead of using the grid's background color.

I'm using PNGs and I'm fairly certain that the files are fine. In fact, if I set the exact same image on an roPosterScreen then the transparent parts show up as white instead of black. So the Roku seems to understand the transparency, but I can't figure out how it's deciding what to composite the image on top of.

Any ideas what I'm doing wrong? I'm happy to include more information, attach files, whatever.