Pasting images from Clipboard into a form doesn't work.
This is not a documented feature within ServiceNow. Pasting attachments from the clipboard directly into the forms has not been documented as a method of adding attachments in forms in the platform.
If this is seen working somewhere, it could be browser extension from a third party supporting this functionality.
Since this is not a documented feature, it is not expected to work. As mentioned above, should you see this working somewhere, try replicating the behavior in an incognito window and you will see that the functionality does not work.
This has been requested as a feature for upcoming releases and the devs can decide whether to include such a feature in any releases in the future.