A Grim Spectator - Padfoot visits Harry's Quidditch game, accompanied by melodramatic lightning.
Tackle - Sirius (as a dog, of course) knocks Harry down in an attempt to get at Scabbers.
Dangerous Black - Sirius corners Harry & Co. in the Shrieking Shack, closing the door and raising Ron's stolen wand.
A Pummelling - Sirius suffers the brunt of Harry's wrath.
Pummelled - "Going to kill me, Harry?" Yes, Sirius does actually get a nosebleed.  It says so in the book. 
Bodyguard Cat - Crookshanks leaps to the defense of his friend.
Hugged - The Professor walked to Black's side, seized his hand, pulled him to his feet so that Crookshanks fell to the floor, and embraced Black like a brother.
Taking a Breather - Black ... suddenly crossed to the four-poster bed and sank onto it, his face hidden in one shaking hand.  Crookshanks leapt up beside him and stepped onto his lap, purring.
Restrained - ...They need to understand – We've got to explain – "
"We can explain afterwards!" snarled Black, trying to throw Lupin off.  One hand was still clawing the air as it tried to reach Scabbers .
Teamwork - "Of course," said Black, and the ghost of a grin flitted across his gaunt face.  He, too, began rolling up his sleeves.  "Shall we kill him together?"
   "Yes, I think so," said Lupin grimly.
A Grin - In reference to Harry wanting to move in with Sirius: "Yeah, I mean it!" said Harry. Black's face broke into the first true smile Harry had seen upon it. This smile is supposed to hark back to young, happy Sirius, but I think this drawing hints more at manic, loony Sirius.
Enter the Dementors - Sirius is surrounded by dementors by the lake ... uh oh.
Sirius Gets Rescued - "How – how – ?" said Black weakly, staring at the hippogriff.
Family Ties - "I see you've met my mother" – Sirius welcomes the protagonist and his friends to his home.
Looking Menacing - This started out as another take on the Daily Prophet photo but then I added a body. The body still looks added on, to me.
Escaping Azkaban - Sirius (as Padfoot) escapes the dread fortress. This would look better framed vertically, but ... oh well.

Drawing the Curtain - Sirius moves to cover up his mother's portrait. I was surprised, when I drew this, that as soon as I started drawing a character I drew in high school, my drawing skills automatically regressed to that level.
Sirius Cleans House - A gag sketch of Sirius, all done up in housecleaning gear and not very happy about it.
Escape Plan - [Order of the Phoenix spoiler!] James and Sirius plot a breakout. See the bottom of the page for a fuller description.

Padfoot and Moony - Drawn mostly to contrast the head shapes of a wolf and a dog, and to illustrate why Padfoot should be a big burly square-faced dog rather than a black wolf-shaped one.

The Daily Prophet Photo II - After I designed young Sirius I went and redid the Daily Prophet photo to better fit the facial structure.
Peter's Plea - I think this is the most frightening version of Sirius I've ever drawn, but not in a good way.
The Door - Sirius sits fuming on the bed in the Shrieking Shack when the door mysteriously opens 'by itself.'
Hey, Nice Hair - Bill greets Sirius by referring to one of the few things they have in common ... long hair.
The Daily Prophet Photo I - I'd tried drawing this about a dozen times before, but this is the first one that looked presentable.
Sirius Afrighted - Sirius is frightened by something. He's got short hair so it must be sometime in Goblet but I honestly don't remember.
Confronting Lupin - Sirius and James figure out what Lupin is and confront him about it (an imagined scene).
Pitch Black - When your name is a word, you are vulnerable to visual puns.
The Potter Wedding - An attempt at drawing the photo in Harry's album, with Sirius at James and Lily's wedding. I have never been really happy with it.