A bit off topic but let's go:

I'm not 100% sure about this but I think I heard from the leader of Lenin-museum at Tampere, Finland that actually Trotsky wasn't in the picture at the first place. But the leaders of revolution (i.e. coup d'├ętat) thought it wouldn't be appropriate for Trotsky not to appear alongside Lenin in the famous picture and therefore he had to be added. So what stalinists did was actually just to erase Trotsky and thus restore the original picture.