Try dragging the photo to the desktop and attaching from there, make sure send Windows friendly attachments. Under edit>attachments, click always send Winows friendly attachments. hey should show up as an icon at the other end.
It is an attachment. Mail displays the image for an attached picture file, but it's just an attachment. If the resolution is less than the original image, check the image size pop-up at the bottom of the new message widow. Set it to "Actual Size".