Gmail seems to do some image processing when an image is uploaded, compressing the (JPG) image. There are also options to select the size of the image (small, best fit, original size). Also, there is an option to attach an image via Dropbox.
How can you determine the Gmail quota used when attaching an image?
From my brief inspection with the browser debug tools, it seems the same quota is used regardless of the size ("small", "best fit..."), but I only inspected emails while composing them, not after actually sending them. Also, even if Dropbox is used, it seems the image is copied to Gmail, so Google storage is still used.
Manually compressing the image before attaching with Gmail seems to result in even smaller quota usage.
Also I'm curious if Gmail will also compress non-lossy image types like PNG and GIF.