To be able to do this you will need to detour through Preview. Copy the image from the browser window and create a 'New From Clipboard' document in Preview. Select all of the image, copy it again, close the preview window, and paste it into the e-mail.
The reason for this is that Chrome sets the properties of the Clipboard entry differently and Mail gets confused as to how to display the pasted content.
For keyboard fans this is the following sequence:
Cmd+C (Copy), Cmd+Space (Search), 'Preview', Enter (start Preview), Cmd+N (New from Clipboard), Cmd+A (Select All), Cmd+C (Copy), Cmd+W (close Preview window), Cmd+Tab (switch to Mail), Cmd+V (Paste). Voila!
[crarko adds: I tested this, and it works as described.]

