There's quite a major BUG going on with pasting images from the clipboard.
You write an email on the web interface, you paste an image from the clipboard, it appears as "image.png". You keep writing the email you past another image, it appears again a second image.png file in the email attachments list and on the email body as the actual image.
You then hit send, the email you send has none of those images, despite them being in the email body and on the attachment list.