Gmail now treats embedded images just like image attachments. You can mouse over an image and click "Download" and "Save to Drive" or you can click the image to open it in the Google Drive viewer, which has additional features like printing, Google+ sharing, zooming, opening the file in a Google Drive app and more. It's also great for opening the image in full-screen mode. You can use the arrow buttons or press right/left arrow keys to open the next/previous image.
"Similar to attachments, you can now click on large images within a message to see them full-screen in Gmail on the web," informs Google. It didn't work for a 384 x 103 image and a 266 x 210 image, while it worked for a 300 x 153 image. I assume that the minimum image size required is 300 x 150.
