For me, the help popped up for an instant but then instantly vanished. Does it only appear during upload/init? That process can go quickly…

We only auto-show it during that time. The hope is that it triggers you to click the button if you need it, but lets you ignore it if you already know the drill...

(all these wonderful tradeoffs between "first visit" vs "repeat visit" - and what if it was missed the first time, blabla... )

The animation was a bit too quick to make that behaviour obvious, so I just tried without it the first time. I thought it was a bug.

The second time I loaded an image, it did the same thing, and only then did I actually click Help.

Yeah, same thing for me. The action was too quick for me to notice that it was intentional. I would just leave the help window open on first time use until the user decides to close it. Very nicely done site though!

I love it! A couple of quick Photoshop shortcuts would really seal the deal, like holding down Spacebar to activate the grabber tool.