I measured the on-screen space that my framed images take in Photoshop, and compared it to in-game, and in-game the images are being stretched by about 0.75cm. It happens to all images -- including valve's own framed map images -- and there doesn't seem to be a good solution to this. The result is that my images look lower-quality and rather grainy. I tried the various mipmap/no mipmap etc settings, but you'll note, if you open Valve's own VTF files for objectives-page images in VTFEdit, that they actually include mipmaps; so this doesn't seem to be a genuine factor.
I also tried making a 1024px image, in the hope that downsizing from that would look better. However, because the new size isn't exactly half the size of 1024 (since 512 had to size up, remember) it isn't any cleaner either; presumably the game is taking the 1024px image and scaling it down using the 512px mipmap as a rough sampling base. It doesn't look great either.
I also tried ensuring that the frames+images took up the entire height of the 512px image. I thought that maybe the game was taking the non-transparent part of the image and scaling it up to 512px high. But doing that made no difference -- the image still was scaled up somewhat, reducing the quality.
It's possible, of course, that I was on the right track and it was the _width_ of the frames that needed to be increased slightly. I don't know -- haven't tried that.
Also, it's worth noting that in Valve's images, the frames are halfway along the image, not on the very left-hand-side. Putting them halfway in makes the screen look better and gives the objectives text more room to breathe.
If anyone has further info on this, suggestions, or comments, please write here.