You are correct, this behavior is not supported as of now. The {clipboard} command only supports rich text or plain text formats (no binary data like images). And, the extension cannot emulate the paste key combination.
Can you put the screenshot directly into your snippet, and then run the snippet? Can you give us some more information about your specific workflow - how you need to take a screenshot and then run the snippet - so that we can better assist you 
This was a related request that came up some time ago: Inserting Screenshots with {clipboard}