I have tried various image paste WordPress Plugins but always come across some drawback or other. Imagepaste works well but puts the image in wp-content\uploads with an unrecognisable random file name and does not add the name to the Media library. Image-elevator looks good but forces you to use the premium (paid for) version to get any real value out of it.
However I have discovered a work-around in Kubuntu using the fantastic KSnapshot screen capture program. This allows you to drag and drop the snapshot image directly into WordPress.