HP Art: Binowlculars, and web app rec
I was sad that artpad is still broken and shows no signs of ever working again, but then I found Queeky. It's buggy and the ToS is lulzy, but maybe those things will be worked out.
It has gradient fills. GRADIENT FILLS. With opacity controls. (Except a new beta went live recently, and some things haven't been upgraded yet, so I'd recommend the old tool: http://www.queeky.com/draw/php/oldTool.php to start drawing, or append ?pic=number to edit a drawing.) You can replay drawing movies, choose your canvas dimensions, apply CC licenses, and save as bitmap. Or in theory as vector, but the vector save hasn't worked for me yet. Also there's a multi-person collaborative thinger.
Title: Binowlculars
Fandom: Harry Potter
Media: Queeky
Notes: "'Yes,' said Hermione irritably, turning a page of Intermediate Transfiguration and glaring at a series of diagrams showing an owl turning into a pair of opera glasses." -- Harry Potter and the Order of the Phoenix, Chapter 31
Click through to replay movie.
ETA: Or stream here: