Trello detailed some of their shortcuts today on their blog. Only one was new to me, but it's a great one:
Just find an image on the web, right click and select “Copy Image”, open or hover over a card, and press “command + v” (Mac) or “control + v” (Windows). It works with any image on your clipboard.
It mostly works great, if a tad slow. You'll see an upload progress indicator in the top right if it's a largish image, and a notification that the image is being processed before it pops up on your card.
Pasting an image from farther down in my clipboard manager (I use ClipMenu ) worked as well. With the way Trello uses the first attached images as a "cover image" in the main view, you could add an icon or custom label onto many cards quickly using the hover-paste technique and clipboard snippets.
Another possibility: capture with Skitch, immediately ⌘+ C, then ⌘+V over a card to attach a screenshot quickly.
Now I want to be able to hover over a card and paste text from my clipboard as a comment.