PNG thumbnail images with transparency?

On my web sites, article thumbnails can be either JPEG format (if photographic) or PNG format (if line art like logos). Some of the PNGs also support transparency, since I’d like for logos to be rendered smoothly against the background color of the page.

But when WordPress resizes the image, it creates a JPEG (and no transparency, since JPEG doesn’t support it). Is there any way to get it to keep the file types consistent when creating image sizes, and to preserve transparency where it exists? Maybe even a hook somewhere where it can be told that, if there’s transparency in this particular image, to save the thumbnail size as PNG instead of JPEG?

PS: If there are hooks that might let me do this myself via a plugin, let me know! If I get it working I’ll post my solution.