Here's what I've got so far:

OS X: exec open myphoto.jpg; exec open http://wiki.tcl.tk

Windows: eval exec [auto_execok start] myphoto.jpg; eval exec
[auto_execok start] http://wiki.tcl.tk

Linux/Unix (Gnome): exec gnome-open myphoto.jpg; exec gnome-open
http://wiki.tcl.tk

Linux/Unix (KDE): exec "kfmclient exec myphoto.jpg"; exec "kfmclient
exec http://wiki.tcl.tk"

Linux/Unix (other): tk_getOpenFile ("Please choose the program to open
this file":)

Two questions:

1. Are these the right commands to use, particularly on Linux, or are
there others (kstart?).
2.  Am I missing anything major from the X11 world, i.e. other desktop
environments that have an API/system command for launching applications?
 If so, can anyone point me in the right direction?

Thanks.

  eval exec [auto_execok start] [list "" "my photo.jpg"]

a) You need a "" for the "title" argument to START (check out
START/?).  Otherwise, when the first argument is quoted, START will
treat that as a title.  And the filename will be quoted when it
contains spaces.  This is on NT/W2K, I don't know about W9x/Me.

b) You need [list] to offset the [eval] when your you do a) and/or
when your filename contains spaces.

The easiest on Windows is probably to write or re-use a binary
extension that wraps the ShellExecute API, like the one on
e.g. <http://wiki.tcl.tk/13925>.