I'm glad you're enjoying the art here. It makes me want to go back to the fandom somewhat (and try to update that LJ some more! Last entry was what, six months ago? Oops.)
Also, it looks like I really should have a look at the French part of the fandom, it seems interestingly active these days.
As for preferences, with Sirius alone in the picture... Heh, I'm not sure I can pick up one picture, but here are a few I like...
Two portraits that are the most fandom-ish, I think.
Sirius on a bike
Sirius in dog form (heh, he's still Sirius, isn't he?)
There's also the Sirius in Azkaban one which you can find at my Elfwood page. Largely creepy, and also somewhat awkward since it's one of the first watercolour painting I ever did... Still, I like it! If you wish to rec it, though, could you please rec it separately from my other stuff, and not like my lj name with my Elfwood name? I'm a bit wary of the connexion being done, especially on a French-speaking forum - not sure all my colleagues and students would understand about fanart and slash, ahem.
As for your second point, my default icon is from that very old piece of fanart, which was an illustration for one of casirafics fanfic. I'm not so proud of the whole painting now than I was when I made the icon... But I still like the icon!