In Mac Mail on OSX 10.11 is it possible to reduce the image size using the inbuilt image size tool (see screenshot below) without it effecting any images that may be in my footer. So that it would only reduce the image size of attachments ?
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.