That's hard to say. Saving the 512x512 FireFox logo, for example (with white background for fair comparison) at 99% quality JPEG from GIMP saves less than 5% sp

That's hard to say. Saving the 512x512 FireFox logo, for example (with white background for fair comparison) at 99% quality JPEG from GIMP saves less than 5% space over PNG, and for images with even more solid white and simple gradients, PNG will beat high quality JPEG. Even more importantly, frequency-domain compression is really not a very good alternative for most alpha channels for two main reasons. First, most alpha channels are dominated by large regions of full transparency or full opacity. PNG thrives under those conditions. JPEG does OK. Second, ring artifacts are a big problem for alpha channels. Whereas a ring artifact in a chroma channel will often be completely imperceptible to the viewer, a ring artifact on an alpha channel can turn transparent pixels opaque. Unless you correct the non-alpha channels to account for those artifacts on the alpha channel, you'll end up with noticeable black halos, even at relatively high quantization. That correction is difficult, and even if you do it well, the results are not stellar. So while a hybrid format might work well, JPEG + alpha is not as simple as it sounds.