Apply "Reduce Image Size" in mac mail to attachments only, not footer
Will work for pictures that you have dragged and dropped onto the mail message. Apple Mail features a selector with small, medium and large options. see selector to the upper right of the image.



Thunderbird lets you click on the image then you can resize the image, but a smaller image doesn't reduce the file size.

If you are desperate for this functionality, you may be able to write an applescript to adjust the picture size. A lot of assembly required.