Let me say that this may not work with all browsers, for me at least it doesn’t work with Safari. Proceed with these steps:

•  Select the image you want to include from a webpage (so that it becomes highlighted)
• Copy to clipboard (ctrl+c , cmd+c, right click…)
• Switch to gmail
• Enable rich formatting
• Paste (ctrl+v, cmd+v, right click…)
• You’re done!

The image will be embedded in html code now, on gmail you’ll be able to view it by selecting for received images Display images below