Google is my friend. The first time I tried this, it didn't work. However, after I learned to read, it worked.
Image must be hosted somewhere on the web.
If the image appears on a page together with other items:
Click on the image with the right mouse button.
Select View Image from the menu.
Hit Ctrl-A (Windows, Linux) or Command-A to highlight the image.
Hit Ctrl-C (Windows, Linux) or Command-C.
Make sure rich text editing is turned on in the Gmail message you are composing.
If you can see a Rich formatting » link above the editing area, click it.
Position the cursor in the Gmail message where you want the image to appear.
Hit Ctrl-V (Windows, Linux) or Command-V.
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