Transparency in image filled nodes

[<mauricio0copo AT gmail.com>]

Hello Graphstream Mailing List!
So, my application needs transition effects when changing graphs, and nothing
could be better than to change transparency. That's easily achieved with 
edges,
but not with nodes... All my nodes are being "filled" by images (fill-mode:
image-scaled), therefore I couldn't find a way to change its transparency. Is
there any way to do that?