Discovering image dimensions

[Travis Reitter]

If a large page (like Planet GNOME) has a handful of images in some of
the top posts, and you scroll down to read another post, what you're
trying to read can keep getting bumped down if the images do not have
set <img> dimensions.

Of course, it's "the blogger's fault" for not setting the image
attributes, but I think it'd be handy if Planet Planet detected (via
ImageMagick - there are Python bindings) image dimensions, and set them
within each feed (if they don't already exist).

Maybe this wouldn't be a default (to not increase minimum dependencies),
but it'd be a nice option.

-Travis