Show a color picture as part of subtitles

[cdlvcdlv]

I would like to use a 24 bits color picture as part of a subtitle stream in a mkv that would be played correctly at least in Windows with DS player (MPC-HC) and VLC. It would be great that worked in Android too (with MX Player).

I think there are two kinds of subtitles that would suit me: ASS and USF. Although USF seems the answer, also nobody uses them. No editor but, well, I think I could write a test by hand to see the possibilities.

I got to this webpage, with that enigmatic sentence:
http://www.matroska.org/technical/specs/subtitles/index.html

Because a "subtitle" stream is actually just an overlay stream, anything with a transparency layer could be use, including video.

Close to that, this one:
http://www.matroska.org/technical/specs/subtitles/ssa.html

Wav: 0, 128697,D:\Alex\Anime\- Fansub -\- TAFF -\Wolf's Rain\WR_-_02_Wav.wav
Wav: 0, 120692,H:\team truc\WR_-_02.wav
Wav: 0, 116504,E:\sub\wolf's_rain\WOLF'S RAIN 02.wav
LastWav: 3

And the "full" specifications of ASS subtitles (http://moodub.free.fr/video/ass-specs.doc) states that you can write in addition to Dialogue events, Picture and Movie events. Pictures can be uuencoded (that I would prefer to avoid) or as an external file (maybe embedded in mkv just as you do with a needed font in this format?).

I've tried following the instructions to no avail to make appear a movie or picture (png) with an ASS subtitle.

Also, I have seen ASS scripts made with Aegisub that use pictures as part of a karaoke effect. Although they work inside Aegisub, the effects aren't seen in MPC-HC with VSFilterMod. I think fansubbers use this scripts to hardsub, which is not what I want. I want to add a subtitle track that you are able to turn on and off.

In short, is there a way to add a color picture as part of subtitles? Could anyone make a "Hello world I am a picture in a subtitle" sample that work with any combination of OS/player/filter(s)/sub format?

TIA