A 320x240 image is created on upload, which is the same size on the /manage/pic page, so it'll be sure to be used. But also, any thumbnails are scaled using the 320x240 as a base.
Actually, the rule is... if it wants to scale to W*H, it needs to find a version of the original that is at least W*H*4 pixels. I figure that will guarantee a high-quality scale (minimum 2x2 averaging regions), but maybe that's overkill. We'll see. It's better than the slowness before, though.
But as thumbnails are 150x150 max now, 320x240 is large & small enough to be the source for any uncropped, unzoomed thumbnail.
The fun part is later when we have to decide which scaled version to start with when we also have a cropping region. :-)