Quicklook is what you get if you press Space in Finder to view the content of a file
Preview.app is an application on its own
Unless I misunderstood your intention I don't think these two tags should get merged.
Am I missing something?
Only that you need to save the file to disk before uploading it to the site. The browser can't upload the contents of your system clipboard to imgur, which is where images end up getting hosted for the StackExchange network.
When it says "paste or drag-and-drop" they mean you can paste a path to a file on your hard drive. When you ...
Here's how I'd like this StackExchange feature to work (noted here as suggested by @bmike, since the question is now tagged as a bug):
Start writing a question or answer on StackExchange in Safari on OS X.
Use cmd-shift-ctrl-4 to take a screenshot and put it into the clipboard.
Press the Image button at the top of the StackExchange editor window.
If it doesn't work, this sounds like a Safari issue. Running those steps just worked in Chrome. After I hit paste, the dialog box changed to this:
Which allowed me to click confirm and upload the image. You may want to click within the whitespace of the dialog box in order to ensure that it has focus, and then hit ⌘v