Its up to theme authors. Option to upload themes is there, all they have to do is upload it. So if you want a specific theme, poke its author.
There is a small catch though: theme has to pass automatic validation. I've noticed that many SMF themes have invalid images, including 1 invalid image in Curve theme (images/im_switch.gif was incomplete: file ended in middle of local palette block, but I've replaced it with fixed image before uploading). Some other themes have png images with .gif extension. It usually can be fixed by simply opening and then saving (better use "save for web" option, so it would optimize image) image in Photoshop.