How can I paste a screenshot image from clipboard into a folder?
I created an automator workflow to solve this problem 

Bottom line: the solution allows you to save an image from the clipboard anywhere with one keystroke (you have to show it where to go in the save dialogue, obviously)

The workflow opens preview, creates a new file from the clipboard, and saves the file (opening the save as dialogue). You can then put it wherever you want. Here's the link to a zip with the workflow. To run the workflow, you'll probably need to approve automator with certain permissions (it'll pop up if and ask for approval, so don't worry if you don't run into this problem). Open the zip, then double click the file (make sure there's an image in the clipboard). A new file should open in preview and ask to be saved.

To create a shortcut, just go to 'keyboard' under system preferences > 'shortcuts' > 'services' and setup the shortcut.