Salamandre wrote:
Well, the new image has the right shape and 0.255.255 cyan around so it should be transparent. The result is fantastic

:
Any idea on this procedure?
I think that photoshop messed with your color table. Anyway I used GIMP for this, its simplicity is best to prevent your results being screwed.
Looking at one of my images:
cyan 0,255,255
"shadow" 210,180,110
256 colors in table, 2 used, first one is the cyan and on the position of 96 is the shadow color. That I think should not matter but first one and perhaps a few behind them could. Especially the first color - there is h3 engine sensitive.
And I think there is way how to prevent "optimizing" color tables in photoshop or photopaint however I don't remember. (Owning Corel so used photopaint in the past.)