I suspect that if the picture was taken with a modern smart phone or camera, it will register the orientation at the time the picture was taken and store this information within the picture as so-called exif data. Specialised graphics viewers will read this information and use it to automatically present the image rotated to suit. Thunderbird is not specifically an image viewer and doesn't look at the exif data. You should be able to view the image in a graphics program and re-save it in its rotated orientation. Now you can send the corrected version of it by email knowing that it is intrinsically "the right way up" and seeing it properly won't depend on the happy accident of the recipient having a smart viewer to auto-rotate it.