That is an older version of the documentation page (we rev the developer documentation with every feature release). The
latest version is here. However, it doesn't include image thumbnails. One small change it has is that the selectors that don't currently have a custom image are broken out into their own table (there are 2 tables).
Custom plugin images are definitely still on the request list. It is a complicated feature since it requires changes to the Indigo Server, Web Server, Client, as well as Indigo Touch. It is high on my priority list (which unfortunately doesn't always map to a "short term" timeline), along with plugin-specified control UI.