I don't think you can ctrl+V (paste) screenshots into those. This app allows you to do just that for screenshots. It's not really for file sharing.
CloudApp: CTRL + OPTION + C
Uploads contents of clipboard.
You can also drag/drop to the icon in the menubar, as well as auto-upload after ⌘ + SHIFT + 3/4.
Ah ok, it appears then ScreenBin differentiates from CloudApp because it doesn't require any installing/downloading of an app to utilize. It's completely in browser.