Glad you asked; I've been meaning to do a writeup on custom type icons.
You've roughly done the right thing -- create a 16x16 png named as
type_unique_name.png.
There is a more "proper" way to do it, though. Basically, along with
this UI change we've introduced a new concept to Reason -- a local
Reason www directory -- that we recommend using for this purpose.
The idea is that you can set up a local directory of web-available
files that overlays Reason's core web-available files. It works in
essentially the same way the core/local code split does: a requested
file will be served up from the local directory if the file exists
there; otherwise things fall through to the core directory.
To set this up. you can just re-run the Reason setup script; it should
create the appropriate .htaccess file and create a directory at
reason_4.0/www/local/. If you prefer, as we at Carleton do, to manage
your local files outside reason_package, you can drop a symlink (or,
in Windows, a junction) at reason_4.0/www/local/ to wherever you'd
like to manage your local web-available files.
Once you have that set up, put your image(s) in
reason_4.0/www/local/ui_images/types/ and you'll see your images show
up. It's a little more complex but it helps keep the core assets and
the local assets distinct from each other, and even gives you the
option of replacing core files with local ones if you think that's
important.
Also of note: the type icons are semitransparent pngs, so if you have
anyone using ie6 the icons will look a bit "off" to them. There is a
way to make things look nicer for them, and we may put that in place
at some point, but if nobody complains I'm not rushing to do it. :)
Matt