Cleanup gui images

Second attempt... first one was not platform-independent

I am a total tcl/tk newbie, but I found it odd to have base64-encoded images embedded inside the GUI code...

If this is by design, please ignore this pull request :)

- Remove wfpxsm, which is unused
- Take bas64-encoded images out of gorilla.tcl, into separate GIF images in pics/ (GIF because guaranteed to be supported by tk).