probably just not waiting long enough for the image to complete downloading. My boss keeps complaining about SMS or MMS messages having low quality images and they are because he is viewing them on his phone immediately the message is received and the software starts displaying the image before the whole image file is downloaded. Perhaps the same is happening here?

I can see no way for Thunderbird, without the help of third party products like an antivirus scanner or an addon to "shrink" images, to reduce the image to anything but what it is. You click attach and the file is converted to MIME encoded text the same as it has been for the last 20 or 30 years, when the recipients get the email the MIME text is converted back into an image.

Sorry, but you are thinking completely wrong. 1. Sending from PC, not mobile device 2) If the email was sent and arrived in terrible quality, I see the same terrible quality when I open this email in my "Sent" Thunderbird folder

I can see no way for Thunderbird, without the help of third party products like an antivirus scanner or an addon to "shrink" images, to reduce the image to anything but what it is. You click attach and the file is converted to MIME encoded text the same as it has been for the last 20 or 30 years, when the recipients get the email the MIME text is converted back into an image.

I have to agree. TB attaches a file to an email and faithfully transmits the file. It has no inbuilt capacity to degrade a file. Unlike when you upload an image, say, on FB. FB deliberately degrades an image before displaying it if it falls outside the parameters - that's the function. TB isn't designed to do that. I don't believe any email program is designed to do that.

One other thing to check is this - is the file size of the attachment that's sent (both on the HD and attached to TB), the same as the file size that is received? If you send a photo in jpg format, what is the dpi of the image in all three locations mentioned previously (for dpi, right click on the image file and go to 'Details' and look for it).