I almost always want mail to send a "Actual Size" image. However sometimes having mail automatically resize an image to "small/medium/large" is handy. However, after doing so Mail remembers the last selected option and sets any future image attachments at that smaller size - which at times, I forget to switch back "to Actual Size". I would like mail to ALWAYS default to "Actual Size" for attached images. — I doubt there is a known workaround but if so, I'd love to know it.
You can select image size below the subject line. There you can find a selection bar for small, middle, large and original size which sends the image without compression.