[DiapDealer]

That's a pretty tall order. When copying web content, there's almost never any actual images in the mimedata contained in the clipboard. There's merely urls. So Sigil would need to intercept all paste signals, parse the clipboard's mimedata for urls, determine which urls are images, download those images to Sigil's scratch/Images directory, and finally update the urls in the clipboard's mimedata with the new local filepath. That's a lot of effort when web content can often be easily saved locally and then imported.