Tk uses a hack for transparency in GIF images.  If you have a GIF89
image that has the transparency index set, you need to set the global
Tcl variable `TRANSPARENT_GIF_COLOR` to the color which you want the
show through (in general, the background of the widget you are placing
it on).  Use the following code as an example:


    label .x
    set TRANSPARENT_GIF_COLOR [.x cget -bg]
    image create photo x -file image.gif
    .x configure -image x

Donal Fellows has a minimal C extension to support real transparency at
http://r8h.cs.man.ac.uk:8000/tcl/transPhoto.c

Tk8 removes the need for this hack, properly implementing transparency
in GIF images (some bugs may still exist).